Escherichia coli Unsaturated Fatty Acid Synthesis

Although the unsaturated fatty acid (UFA) synthetic pathway of Escherichia coli is the prototype of such pathways, several unresolved issues have accumulated over the years. The key players are the fabA and fabB genes. Earlier studies of fabA transcription showed that the gene was transcribed from two promoters, with one being positively regulated by the FadR protein. The other weaker promoter (which could not be mapped with the technology then available) was considered constitutive because its function was independent of FadR. However, the FabR negative regulator was recently shown to represses fabA transcription. We report that the weak promoter overlaps the FadR-dependent promoter and is regulated by FabR. This promoter is strictly conserved in all E. coli and Salmonella enterica genomes sequenced to date and is thought to provide insurance against inappropriate regulation of fabA transcription by exogenous saturated fatty acids. Also, the fabAup promoter, a mutant promoter previously isolated by selection for increased FabA activity, was shown to be a promoter created de novo by a four-base deletion within the gene located immediately upstream of fabA. Demonstration of the key UFA synthetic reaction catalyzed by FabB has been elusive, although it was known to catalyze an elongation reaction. Strains lacking FabB are UFA auxotrophs indicating that the enzyme catalyzes an essential step in UFA synthesis. Using thioesterases specific for hydrolysis of short chain acyl-ACPs, the intermediates of the UFA synthetic pathway have been followed in vivo for the first time. These experiments showed that a fabB mutant strain accumulated less cis-5-dodecenoic acid than the parental wild-type strain. These data indicate that the key reaction in UFA synthesis catalyzed by FabB is elongation of the cis-3-decenoyl-ACP produced by FabA. Although the unsaturated fatty acid (UFA) synthetic pathway of Escherichia coli is the prototype of such pathways, several unresolved issues have accumulated over the years. The key players are the fabA and fabB genes. Earlier studies of fabA transcription showed that the gene was transcribed from two promoters, with one being positively regulated by the FadR protein. The other weaker promoter (which could not be mapped with the technology then available) was considered constitutive because its function was independent of FadR. However, the FabR negative regulator was recently shown to represses fabA transcription. We report that the weak promoter overlaps the FadR-dependent promoter and is regulated by FabR. This promoter is strictly conserved in all E. coli and Salmonella enterica genomes sequenced to date and is thought to provide insurance against inappropriate regulation of fabA transcription by exogenous saturated fatty acids. Also, the fabAup promoter, a mutant promoter previously isolated by selection for increased FabA activity, was shown to be a promoter created de novo by a four-base deletion within the gene located immediately upstream of fabA. Demonstration of the key UFA synthetic reaction catalyzed by FabB has been elusive, although it was known to catalyze an elongation reaction. Strains lacking FabB are UFA auxotrophs indicating that the enzyme catalyzes an essential step in UFA synthesis. Using thioesterases specific for hydrolysis of short chain acyl-ACPs, the intermediates of the UFA synthetic pathway have been followed in vivo for the first time. These experiments showed that a fabB mutant strain accumulated less cis-5-dodecenoic acid than the parental wild-type strain. These data indicate that the key reaction in UFA synthesis catalyzed by FabB is elongation of the cis-3-decenoyl-ACP produced by FabA. The fabA gene of Escherichia coli encodes 3-hydroxydecanoyl-acyl carrier protein (ACP) 2The abbreviations used are: ACPacyl carrier proteinUFAunsaturated fatty acidSFAsaturated fatty acidRTreverse transcriptaseFFAfree fatty acid5′-RACE5′-rapid amplification of cDNA ends. 2The abbreviations used are: ACPacyl carrier proteinUFAunsaturated fatty acidSFAsaturated fatty acidRTreverse transcriptaseFFAfree fatty acid5′-RACE5′-rapid amplification of cDNA ends. dehydratase/isomerase, a bifunctional enzyme that introduces the unsaturated fatty acid (UFA) double bond at the C10 level (1Bloch K. The Enzymes.in: Boyer P.D. 3rd Ed. Academic Press, New York1971: 441-464Google Scholar, 2Cronan Jr., J.E. Li W.B. Coleman R. Narasimhan M. de Mendoza D. Schwab J.M. J. Biol. Chem. 1988; 263: 4641-4646Abstract Full Text PDF PubMed Google Scholar, 3Cronan Jr., J.E. Gelmann E.P. J. Biol. Chem. 1973; 248: 1188-1195Abstract Full Text PDF PubMed Google Scholar) (see Fig. 1). As expected fabA mutants require supplementation with UFA for growth (4Silbert D.F. Vagelos P.R. Proc. Natl. Acad. Sci. U.S.A. 1967; 58: 1579-1586Crossref PubMed Scopus (99) Google Scholar). Although UFA synthesis can be considered a housekeeping function, fabA transcription is subject to an unexpectedly complex regulation. Work from this laboratory and others showed that fabA transcription is positively regulated by FadR, a protein that also functions as the repressor of the β-oxidation regulon (5Henry M.F. Cronan Jr., J.E. J. Mol. Biol. 1991; 222: 843-849Crossref PubMed Scopus (86) Google Scholar, 6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar, 7DiRusso C.C. Metzger A.K. Heimert T.L. Mol. Microbiol. 1993; 7: 311-322Crossref PubMed Scopus (73) Google Scholar). In that work the FadR-dependent promoter was identified as well as a second weaker promoter that was unaffected by FadR and thus was considered constitutively active (5Henry M.F. Cronan Jr., J.E. J. Mol. Biol. 1991; 222: 843-849Crossref PubMed Scopus (86) Google Scholar, 6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). The FadR-independent promoter is thought to ensure continued expression of fabA under conditions where FadR activation is compromised. E. coli fabA mutants lyse in the absence of UFA (3Cronan Jr., J.E. Gelmann E.P. J. Biol. Chem. 1973; 248: 1188-1195Abstract Full Text PDF PubMed Google Scholar). Because FadR responds to CoA thioesters of both unsaturated and saturated fatty acids, E. coli could shut down UFA synthesis in response to saturated fatty acid (SFA) availability, a potential disaster. We have proposed that E. coli avoids this calamity by the presence of the weaker promoter (8Cronan Jr., J.E. Subrahmanyam S. Mol. Microbiol. 1998; 29: 937-943Crossref PubMed Scopus (102) Google Scholar). This promoter has an activity equivalent to that of the main promoter in the absence of FadR activation (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). The basal transcription initiated at the FadR-regulated promoter plus the weak constitutive promoter accounts for the UFA synthesis remaining in FadR strains. FadR strains have about 30% less unsaturated fatty acid than wild-type cells (9Nunn W.D. Giffin K. Clark D. Cronan Jr., J.E. J. Bacteriol. 1983; 154: 554-560Crossref PubMed Google Scholar) and because a 10-fold decrease in FabA levels produces only a modest decrease in UFA synthesis, wild-type cells must produce FabA in excess. Hence, we that this promoter insurance against of UFA synthesis by saturated fatty of fabA transcription has recently second protein FabR was to fabA and fabB J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, 29: PubMed Scopus Google Scholar). In the work the weak fabA promoter could not be mapped to of the then this the that FabR function of this We report that this is the was the of the in a isolated by selection for increased FabA activity D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar). The increased fabA transcription was shown to a promoter located well upstream of the fabA to a of the constitutive promoter that increased its We report that this is The fabAup is a deletion within the gene located immediately upstream of fabA that in of a in E. coli unsaturated fatty acid synthesis is the reaction catalyzed by the FabB of in UFA Jr., J.E. Vagelos P.R. J. Bacteriol. PubMed Google Scholar). FabB is a well a of that catalyze the elongation reaction of fatty acid synthesis Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar). E. coli has such and J. Escherichia coli and and R. J. J. D. Press, Scholar). The UFA of strains lacking FabB that this enzyme catalyzes a step in UFA synthesis that be by is not a because it only short chain has been that the elongation step catalyzed by FabB is of cis-3-decenoyl-ACP produced by the FabA to Cronan J.E. PubMed Scopus Google Scholar) 1). However, this was negative because in studies and of FabB and have to the of mutants lacking enzyme Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, K. PubMed Scopus (73) Google Scholar, J. PubMed Scopus Google Scholar). We this in vivo by the intermediates of UFA synthesis thioesterases active short chain Cell. 7: PubMed Scopus Google Scholar). the of the fatty acid synthetic J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) we that the by be cis-5-dodecenoic acid and has not previously been to cis-5-dodecenoic acid in E. coli because this acid is not the complex of the only of acid are in the E. coli elongation is by Cronan Jr., J.E. 1973; PubMed Scopus Google Scholar). We report that expression of a of a in of both of short chain de novo fatty acid synthesis of the step catalyzed by These data indicate that the key reaction in UFA synthesis catalyzed by FabB is elongation of the cis-3-decenoyl-ACP produced by that fabA is transcribed from two promoters, both of modest regulation by the FabR repressor protein. the upstream promoter is regulated by FadR with the of the protein as a that at the of the promoter (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). We the (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar) that the fabAup mutant promoter was an of a promoter was The promoter from a deletion that created a promoter from an and is not transcription of a housekeeping is of two and is to at two FadR regulation a to decrease UFA synthesis are in the are to CoA that FadR and it from the promoter in fabA transcription (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). However, the regulation a chain is to the then can E. coli UFA synthesis in the presence of exogenous provide the by fabA expression in the absence of FadR activation (8Cronan Jr., J.E. Subrahmanyam S. Mol. Microbiol. 1998; 29: 937-943Crossref PubMed Scopus (102) Google Scholar). However, this is by the weak of The FabR is and thus the of the protein is and the of FabR to not can be However, the of the promoter and the FabR is by its in all E. coli and genomes in as well as in all Salmonella enterica plus of strains of and The that the has been the the of E. coli and S. enterica that promoter an in fabA of thioesterases specific for short chain has the intermediates of UFA synthesis to be followed in vivo for the first time. that strains in FabB activity a of UFA intermediates to that by a strain in FabA activity that the two catalyze at the level of the the synthetic in FabB strains was of then cis-5-dodecenoic acid have accumulated at the of the UFA 1). in mutants lacking the other E. coli chain are in elongation of to Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, E.P. Cronan Jr., J.E. J. Bacteriol. PubMed Google Scholar). of the mutant strain than wild-type E.P. Cronan Jr., J.E. J. Bacteriol. PubMed Google Scholar). However, of cis-5-dodecenoic acid in the of the strain was the levels than of the wild-type strain the in fabB strains must be to the synthesis of (see Fig. 1). the catalyzed by FabA and the that FabB is a it that FabB catalyzes the elongation of cis-3-decenoyl-ACP produced by FabA to the first fatty acid is to 1). The strain cis-5-dodecenoic acid than such as This can be to the of the that the and intermediates not J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). is that thioesterases are thought to have for of saturated Cell. 7: PubMed Scopus Google Scholar) and thus active be to and be expected that because cis-3-decenoyl-ACP levels of However, only levels of acid by The of of this acid is by the that all of the catalyzed by FabA are (1Bloch K. The Enzymes.in: Boyer P.D. 3rd Ed. Academic Press, New York1971: 441-464Google Scholar, PubMed Scopus Google Scholar) 1). the of the cis-3-decenoyl-ACP be to The be by the and the chain the synthetic it has been shown that of FabA the levels of than the levels of UFA in vivo D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar). This was both FabA and FabB D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar) indicating that FabB is the step in UFA synthesis and cis-3-decenoyl-ACP produced by FabA be to the saturated fatty acid synthetic pathway D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar). also be that the chain of the from the strain are than by the wild-type and strains (see Fig. This is to be expected because the strain have less than of the level of chain activity Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Vagelos P.R. J. Biol. Chem. Full Text PDF PubMed Google Scholar). fabB strain has only the elongation activity, the other strains both and elongation the that the an its elongation to chain where it to of the chain complex of FabB activity in of the the strain accumulated the chain at the of the strain accumulated The fabA gene of Escherichia coli encodes 3-hydroxydecanoyl-acyl carrier protein (ACP) 2The abbreviations used are: ACPacyl carrier proteinUFAunsaturated fatty acidSFAsaturated fatty acidRTreverse transcriptaseFFAfree fatty acid5′-RACE5′-rapid amplification of cDNA ends. 2The abbreviations used are: ACPacyl carrier proteinUFAunsaturated fatty acidSFAsaturated fatty acidRTreverse transcriptaseFFAfree fatty acid5′-RACE5′-rapid amplification of cDNA ends. dehydratase/isomerase, a bifunctional enzyme that introduces the unsaturated fatty acid (UFA) double bond at the C10 level (1Bloch K. The Enzymes.in: Boyer P.D. 3rd Ed. Academic Press, New York1971: 441-464Google Scholar, 2Cronan Jr., J.E. Li W.B. Coleman R. Narasimhan M. de Mendoza D. Schwab J.M. J. Biol. Chem. 1988; 263: 4641-4646Abstract Full Text PDF PubMed Google Scholar, 3Cronan Jr., J.E. Gelmann E.P. J. Biol. Chem. 1973; 248: 1188-1195Abstract Full Text PDF PubMed Google Scholar) (see Fig. 1). As expected fabA mutants require supplementation with UFA for growth (4Silbert D.F. Vagelos P.R. Proc. Natl. Acad. Sci. U.S.A. 1967; 58: 1579-1586Crossref PubMed Scopus (99) Google Scholar). Although UFA synthesis can be considered a housekeeping function, fabA transcription is subject to an unexpectedly complex regulation. Work from this laboratory and others showed that fabA transcription is positively regulated by FadR, a protein that also functions as the repressor of the β-oxidation regulon (5Henry M.F. Cronan Jr., J.E. J. Mol. Biol. 1991; 222: 843-849Crossref PubMed Scopus (86) Google Scholar, 6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar, 7DiRusso C.C. Metzger A.K. Heimert T.L. Mol. Microbiol. 1993; 7: 311-322Crossref PubMed Scopus (73) Google Scholar). In that work the FadR-dependent promoter was identified as well as a second weaker promoter that was unaffected by FadR and thus was considered constitutively active (5Henry M.F. Cronan Jr., J.E. J. Mol. Biol. 1991; 222: 843-849Crossref PubMed Scopus (86) Google Scholar, 6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). The FadR-independent promoter is thought to ensure continued expression of fabA under conditions where FadR activation is compromised. E. coli fabA mutants lyse in the absence of UFA (3Cronan Jr., J.E. Gelmann E.P. J. Biol. Chem. 1973; 248: 1188-1195Abstract Full Text PDF PubMed Google Scholar). Because FadR responds to CoA thioesters of both unsaturated and saturated fatty acids, E. coli could shut down UFA synthesis in response to saturated fatty acid (SFA) availability, a potential disaster. We have proposed that E. coli avoids this calamity by the presence of the weaker promoter (8Cronan Jr., J.E. Subrahmanyam S. Mol. Microbiol. 1998; 29: 937-943Crossref PubMed Scopus (102) Google Scholar). This promoter has an activity equivalent to that of the main promoter in the absence of FadR activation (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). The basal transcription initiated at the FadR-regulated promoter plus the weak constitutive promoter accounts for the UFA synthesis remaining in FadR strains. FadR strains have about 30% less unsaturated fatty acid than wild-type cells (9Nunn W.D. Giffin K. Clark D. Cronan Jr., J.E. J. Bacteriol. 1983; 154: 554-560Crossref PubMed Google Scholar) and because a 10-fold decrease in FabA levels produces only a modest decrease in UFA synthesis, wild-type cells must produce FabA in excess. Hence, we that this promoter insurance against of UFA synthesis by saturated fatty acids. carrier protein unsaturated fatty acid saturated fatty acid fatty acid amplification of cDNA ends. carrier protein unsaturated fatty acid saturated fatty acid fatty acid amplification of cDNA ends. However, of fabA transcription has recently second protein FabR was to fabA and fabB J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, 29: PubMed Scopus Google Scholar). In the work the weak fabA promoter could not be mapped to of the then this the that FabR function of this We report that this is the was the of the in a isolated by selection for increased FabA activity D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar). The increased fabA transcription was shown to a promoter located well upstream of the fabA to a of the constitutive promoter that increased its We report that this is The fabAup is a deletion within the gene located immediately upstream of fabA that in of a in E. coli unsaturated fatty acid synthesis is the reaction catalyzed by the FabB of in UFA Jr., J.E. Vagelos P.R. J. Bacteriol. PubMed Google Scholar). FabB is a well a of that catalyze the elongation reaction of fatty acid synthesis Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar). E. coli has such and J. Escherichia coli and and R. J. J. D. Press, Scholar). The UFA of strains lacking FabB that this enzyme catalyzes a step in UFA synthesis that be by is not a because it only short chain has been that the elongation step catalyzed by FabB is of cis-3-decenoyl-ACP produced by the FabA to Cronan J.E. PubMed Scopus Google Scholar) 1). However, this was negative because in studies and of FabB and have to the of mutants lacking enzyme Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, K. PubMed Scopus (73) Google Scholar, J. PubMed Scopus Google Scholar). We this in vivo by the intermediates of UFA synthesis thioesterases active short chain Cell. 7: PubMed Scopus Google Scholar). the of the fatty acid synthetic J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) we that the by be cis-5-dodecenoic acid and has not previously been to cis-5-dodecenoic acid in E. coli because this acid is not the complex of the only of acid are in the E. coli elongation is by Cronan Jr., J.E. 1973; PubMed Scopus Google Scholar). We report that expression of a of a in of both of short chain de novo fatty acid synthesis of the step catalyzed by These data indicate that the key reaction in UFA synthesis catalyzed by FabB is elongation of the cis-3-decenoyl-ACP produced by FabA. that fabA is transcribed from two promoters, both of modest regulation by the FabR repressor protein. the upstream promoter is regulated by FadR with the of the protein as a that at the of the promoter (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). We the (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar) that the fabAup mutant promoter was an of a promoter was The promoter from a deletion that created a promoter from an and is not transcription of a housekeeping is of two and is to at two FadR regulation a to decrease UFA synthesis are in the are to CoA that FadR and it from the promoter in fabA transcription (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). However, the regulation a chain is to the then can E. coli UFA synthesis in the presence of exogenous provide the by fabA expression in the absence of FadR activation (8Cronan Jr., J.E. Subrahmanyam S. Mol. Microbiol. 1998; 29: 937-943Crossref PubMed Scopus (102) Google Scholar). However, this is by the weak of The FabR is and thus the of the protein is and the of FabR to not can be However, the of the promoter and the FabR is by its in all E. coli and genomes in as well as in all Salmonella enterica plus of strains of and The that the has been the the of E. coli and S. enterica that promoter an in fabA of thioesterases specific for short chain has the intermediates of UFA synthesis to be followed in vivo for the first time. that strains in FabB activity a of UFA intermediates to that by a strain in FabA activity that the two catalyze at the level of the the synthetic in FabB strains was of then cis-5-dodecenoic acid have accumulated at the of the UFA 1). in mutants lacking the other E. coli chain are in elongation of to Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, E.P. Cronan Jr., J.E. J. Bacteriol. PubMed Google Scholar). of the mutant strain than wild-type E.P. Cronan Jr., J.E. J. Bacteriol. PubMed Google Scholar). However, of cis-5-dodecenoic acid in the of the strain was the levels than of the wild-type strain the in fabB strains must be to the synthesis of (see Fig. 1). the catalyzed by FabA and the that FabB is a it that FabB catalyzes the elongation of cis-3-decenoyl-ACP produced by FabA to the first fatty acid is to 1). The strain cis-5-dodecenoic acid than such as This can be to the of the that the and intermediates not J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). is that thioesterases are thought to have for of saturated Cell. 7: PubMed Scopus Google Scholar) and thus active be to and be expected that because cis-3-decenoyl-ACP levels of However, only levels of acid by The of of this acid is by the that all of the catalyzed by FabA are (1Bloch K. The Enzymes.in: Boyer P.D. 3rd Ed. Academic Press, New York1971: 441-464Google Scholar, PubMed Scopus Google Scholar) 1). the of the cis-3-decenoyl-ACP be to The be by the and the chain the synthetic it has been shown that of FabA the levels of than the levels of UFA in vivo D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar). This was both FabA and FabB D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar) indicating that FabB is the step in UFA synthesis and cis-3-decenoyl-ACP produced by FabA be to the saturated fatty acid synthetic pathway D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar). also be that the chain of the from the strain are than by the wild-type and strains (see Fig. This is to be expected because the strain have less than of the level of chain activity Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Vagelos P.R. J. Biol. Chem. Full Text PDF PubMed Google Scholar). fabB strain has only the elongation activity, the other strains both and elongation the that the an its elongation to chain where it to of the chain complex of FabB activity in of the the strain accumulated the chain at the of the strain accumulated that fabA is transcribed from two promoters, both of modest regulation by the FabR repressor protein. the upstream promoter is regulated by FadR with the of the protein as a that at the of the promoter (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). We the (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar) that the fabAup mutant promoter was an of a promoter was The promoter from a deletion that created a promoter from an and is not transcription of a housekeeping is of two and is to at two FadR regulation a to decrease UFA synthesis are in the are to CoA that FadR and it from the promoter in fabA transcription (6Henry M.F. Cronan Jr., J.E. Cell. 1992; 70: 671-679Abstract Full Text PDF PubMed Scopus (124) Google Scholar). However, the regulation a chain is to the then can E. coli UFA synthesis in the presence of exogenous provide the by fabA expression in the absence of FadR activation (8Cronan Jr., J.E. Subrahmanyam S. Mol. Microbiol. 1998; 29: 937-943Crossref PubMed Scopus (102) Google Scholar). However, this is by the weak of The FabR is and thus the of the protein is and the of FabR to not can be However, the of the promoter and the FabR is by its in all E. coli and genomes in as well as in all Salmonella enterica plus of strains of and The that the has been the the of E. coli and S. enterica that promoter an in fabA The of thioesterases specific for short chain has the intermediates of UFA synthesis to be followed in vivo for the first time. that strains in FabB activity a of UFA intermediates to that by a strain in FabA activity that the two catalyze at the level of the the synthetic in FabB strains was of then cis-5-dodecenoic acid have accumulated at the of the UFA 1). in mutants lacking the other E. coli chain are in elongation of to Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, E.P. Cronan Jr., J.E. J. Bacteriol. PubMed Google Scholar). of the mutant strain than wild-type E.P. Cronan Jr., J.E. J. Bacteriol. PubMed Google Scholar). However, of cis-5-dodecenoic acid in the of the strain was the levels than of the wild-type strain the in fabB strains must be to the synthesis of (see Fig. 1). the catalyzed by FabA and the that FabB is a it that FabB catalyzes the elongation of cis-3-decenoyl-ACP produced by FabA to the first fatty acid is to 1). The strain cis-5-dodecenoic acid than such as This can be to the of the that the and intermediates not J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). is that thioesterases are thought to have for of saturated Cell. 7: PubMed Scopus Google Scholar) and thus active be to and be expected that because cis-3-decenoyl-ACP levels of However, only levels of acid by The of of this acid is by the that all of the catalyzed by FabA are (1Bloch K. The Enzymes.in: Boyer P.D. 3rd Ed. Academic Press, New York1971: 441-464Google Scholar, PubMed Scopus Google Scholar) 1). the of the cis-3-decenoyl-ACP be to The be by the and the chain the synthetic it has been shown that of FabA the levels of than the levels of UFA in vivo D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar). This was both FabA and FabB D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar) indicating that FabB is the step in UFA synthesis and cis-3-decenoyl-ACP produced by FabA be to the saturated fatty acid synthetic pathway D. Cronan Jr., J.E. 1983; PubMed Scopus Google Scholar). also be that the chain of the from the strain are than by the wild-type and strains (see Fig. This is to be expected because the strain have less than of the level of chain activity Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Cronan Jr., J.E. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Vagelos P.R. J. Biol. Chem. Full Text PDF PubMed Google Scholar). fabB strain has only the elongation activity, the other strains both and elongation the that the an its elongation to chain where it to of the chain complex of FabB activity in of the the strain accumulated the chain at the of the strain accumulated We of the for with the with with