Mechanisms of Intron Mobility

INTRODUCTIONGroup I and group II introns, which splice via RNA-catalyzed pathways, can invade DNA sequences by virtue of proteins expressed from open reading frames (ORFs) 1The abbreviations used are: ORFopen reading frameRTreverse transcriptaseDSBdouble strand breakDSBRDSB repairSDSAsynthesis-dependent strand annealingBMbubble migration contained within them. These intron products are endonucleases in the case of group I introns and reverse transcriptases (RTs) with associated endonuclease activity in the case of the group II introns. Two kinds of mobility reactions will be considered for each intron type: homing into cognate intronless alleles and transposition to non-allelic sites.Intron HomingIntron homing is a process whereby the intron moves from an intron-containing allele to an intronless allele in a homologydependent gene conversion event. Coconversion of flanking exon sequences often accompanies intron homing (reviewed in (1.Dujon B. Gene (Amst.). 1989; 82: 91-114Crossref PubMed Scopus (394) Google Scholar) and (2.Lambowitz A.M. Belfort M. Annu. Rev. Biochem. 1993; 62: 587-622Crossref PubMed Scopus (534) Google Scholar)).Group I Intron HomingHoming is initiated by the intron-encoded endonuclease, which catalyzes a double strand break (DSB) in the intron-minus (recipient) allele (Fig. 1A). Invasion of the homologous intron donor duplex by a cleaved 3′-end primes repair synthesis that results in copying of the intron into the recipient DNA. This process is thought to proceed via the DSB repair (DSBR) pathway, wherein a D-loop formed as the result of repair synthesis by the invading strand serves as a template for repair synthesis of the opposite strand (Fig. 1B). Two resulting Holliday junctions must be resolved to yield two intron-containing alleles, which would either have an exchange of flanking markers (crossovers), or not (non-crossovers), depending upon the relative direction of resolution of the two junctions. Because of nucleolytic degradation of the cleaved recipient and branch migration, co-conversion of exon sequences flanking the intron is common.Experiments in the T4 phage system have implicated exonucleolytic, synaptic, and DNA-synthetic functions of the phage in the homing process (3.Clyman J. Belfort M. Genes 6: 1269-1279Crossref PubMed Scopus (32) Google Scholar) 2Mueller, J. E., Clyman, J., Huang, Y., Parker, M. M., and Belfort, M.(1996) Genes 20: 387-391Abstract Full Text PDF PubMed Scopus (3) Google Scholar)), which has been invoked as an alternative to the DSBR pathway to explain gene conversion from ectopic sites in P-element-induced gap repair in Drosophila (Fig. 1C)(6.Nassif N. Penney J. Pal S. Engels W.R. Gloor G.B. Mol. Cell. Biol. 1994; 14: 1613-1625Crossref PubMed Scopus (384) Google Scholar). This pathway is similar to the bubble migration (BM) pathway for T4 phage replication(7.Formosa T. Alberts B.M. Cell. 1986; 47: 793-806Abstract Full Text PDF PubMed Scopus (198) Google Scholar). The initial steps of the SDSA (and BM) pathway involving cleavage and strand invasion are the same as those of DSBR. Unlike DSBR, however, Holliday junctions are not formed, obviating the need for resolvase function and resulting only in non-crossover products(5.Shinohara A. Ogawa T. Trends Biochem. Sci. 1995; 20: 387-391Abstract Full Text PDF PubMed Scopus (3) Google Scholar).2 Alternative homing pathways in both pro- and eukaryotic systems require further investigation.Group II Intron HomingRecent studies on the mobility of the group II introns aI1 and aI2 of the COX1 gene of yeast mtDNA establish a number of key features of group II intron homing(8.Lazowska J. Meunier B. Macadre C. EMBO J. 1994; 13: 4963-4972Crossref PubMed Scopus (60) Google Scholar, 9.Moran J.V. Zimmerly S. Eskes R. Kennell J.C. Lambowitz A.M. Butow R.A. Perlman P.S. Mol. Cell. Biol. 1995; 15: 2828-2838Crossref PubMed Scopus (103) Google Scholar, 10.Zimmerly S. Guo H. Perlman P.S. Lambowitz A.M. Cell. 1995; 82: 1-20Abstract Full Text PDF PubMed Scopus (231) Google Scholar). Like homing of group I introns, the process is highly efficient and site-specific. However, there are three important differences from group I intron mobility. First, group II homing depends on an intron-encoded protein with RT activity; second, the process requires a splicing proficient intron, whose RNA processing function in turn relies on maturase activity of the intron-encoded protein; and third, the extent and symmetry of co-conversion are more limited for group II than for group I introns. In group I intron homing co-conversion appears, on average over multiple events, to be roughly symmetric, whereas co-conversion associated with group II intron homing appears asymmetric, extending further upstream than downstream of the intron insertion site.The group II homing pathway (Fig. 2A), which has been elucidated recently for the aI2 intron, is remarkable in that it depends on three activities of the aI2-encoded protein: endonuclease, maturase, and RT(9.Moran J.V. Zimmerly S. Eskes R. Kennell J.C. Lambowitz A.M. Butow R.A. Perlman P.S. Mol. Cell. Biol. 1995; 15: 2828-2838Crossref PubMed Scopus (103) Google Scholar, 10.Zimmerly S. Guo H. Perlman P.S. Lambowitz A.M. Cell. 1995; 82: 1-20Abstract Full Text PDF PubMed Scopus (231) Google Scholar). The endonuclease activity, which also requires the intron RNA, makes a staggered cut in the recipient DNA, cleaving the antisense strand at a specific site in the downstream exon and the sense strand at the junction between the two exons. The 3′-OH of the cleaved antisense strand is used as a primer for first-strand cDNA synthesis by RT, using the pre-mRNA precursor as template(10.Zimmerly S. Guo H. Perlman P.S. Lambowitz A.M. Cell. 1995; 82: 1-20Abstract Full Text PDF PubMed Scopus (231) Google Scholar). This sequence of events is rather analogous to the priming of cDNA synthesis by the site-specific non-long terminal repeat retroelement R2Bm(11.Luan D.D. Korman M.H. Jakubczak J.L. Eickbush T.H. Cell. 1993; 72: 595-605Abstract Full Text PDF PubMed Scopus (896) Google Scholar), consistent with the group II introns representing a type of site-specific retrotransposon.Fig. 2RNA-mediated mobility events. Wavy lines, RNA; straight lines, DNA; thin lines, exons; thick lines, intron. A, group II intron homing. The pathway, worked out for the yeast mitochondrial aI2 intron (8.Lazowska J. Meunier B. Macadre C. EMBO J. 1994; 13: 4963-4972Crossref PubMed Scopus (60) Google Scholar, 9.Moran J.V. Zimmerly S. Eskes R. Kennell J.C. Lambowitz A.M. Butow R.A. Perlman P.S. Mol. Cell. Biol. 1995; 15: 2828-2838Crossref PubMed Scopus (103) Google Scholar, 10.Zimmerly S. Guo H. Perlman P.S. Lambowitz A.M. Cell. 1995; 82: 1-20Abstract Full Text PDF PubMed Scopus (231) Google Scholar) involves the maturase (M), endonuclease (E), and RT activities of the aI2-encoded protein. The activity required for each step is indicated as white lettering on a black background. Endonuclease activity requires intron RNA, which is excised as a lariat but is depicted as a linear molecule. Cleavage sites on the DNA recipient are shown as dots, with the exon junction represented as a line. RT-directed cDNA synthesis is primed from the downstream 3′-OH of the cleaved recipient DNA, with the pre-mRNA as template. Details on completion of the integration step remain to be determined. B, group I and group II intron transposition. Reverse splicing into a foreign RNA, shown to occur for both intron types, yields an RNA that, when reverse-transcribed and recombined with the genome, results in transposition of the intron to an ectopic site. The latter step has some experimental support for group II introns only (22.Mueller M.W. Allmaier M. Eskes R. Schweyen R.J. Nature. 1993; 366: 174-176Crossref PubMed Scopus (85) Google Scholar, 23.Sellem C.H. Lecellier G. Belcour L. Nature. 1993; 366: 176-178Crossref PubMed Scopus (98) Google Scholar, 24.Schmidt W.R. Schweyen R.J. Wolf K. Mueller M.W. J. Mol. Biol. 1994; 243: 157-166Crossref PubMed Scopus (30) Google Scholar). C, group I and group II intron loss. A cDNA copy of the spliced mRNA is proposed to recombine with the genome to render it intron-minus.View Large Image Figure ViewerDownload Hi-res image Download (PPT)This model is in accord with several unexplained observations. First, inhibition of splicing blocks intron homing(9.Moran J.V. Zimmerly S. Eskes R. Kennell J.C. Lambowitz A.M. Butow R.A. Perlman P.S. Mol. Cell. Biol. 1995; 15: 2828-2838Crossref PubMed Scopus (103) Google Scholar, 12.Meunier B. Tian G.-L. Macadre C. Slonimski P.P. Lazowska J. Quagliariello E. Papa S. Palmieri F. Saccone C. Structure, Function and Biogenesis of Energy Transfer Systems. Elsevier Science Publishers B.V., Amsterdam1990: 169-174Google Scholar). The finding that specific defects of the cis-acting intron RNA substructure abolish homing (9.Moran J.V. Zimmerly S. Eskes R. Kennell J.C. Lambowitz A.M. Butow R.A. Perlman P.S. Mol. Cell. Biol. 1995; 15: 2828-2838Crossref PubMed Scopus (103) Google Scholar) can be reconciled with the role of intron RNA in endonuclease function as either cofactor or catalyst(10.Zimmerly S. Guo H. Perlman P.S. Lambowitz A.M. Cell. 1995; 82: 1-20Abstract Full Text PDF PubMed Scopus (231) Google Scholar, 13.Belfort M. Science. 1993; 262: 1009-1010Crossref PubMed Scopus (27) Google Scholar). Second, the model can explain the relatively inefficient co-conversion downstream of the cleavage site as resulting from the limited exonucleolytic degradation that can occur before priming of cDNA synthesis ensues from the downstream 3′-OH of the cleaved recipient. One issue that remains unclear is the manner in which the complement to the first strand cDNA is made. Another is the possibility that group II homing may occur by more than one mechanism. For example, an RT-independent group I-type pathway was suggested by the finding that a mutant aI2 protein that lacks RT activity but retains endonuclease function supports 40% homing activity(9.Moran J.V. Zimmerly S. Eskes R. Kennell J.C. Lambowitz A.M. Butow R.A. Perlman P.S. Mol. Cell. Biol. 1995; 15: 2828-2838Crossref PubMed Scopus (103) Google Scholar, 10.Zimmerly S. Guo H. Perlman P.S. Lambowitz A.M. Cell. 1995; 82: 1-20Abstract Full Text PDF PubMed Scopus (231) Google Scholar). However, further study is needed to determine whether that pathway occurs in wild-type crosses.Intron TranspositionThe sporadic distribution of the conserved group I and group II introns suggests that each arose from ancestral introns that transposed to heterologous sites. One possibility is that the introns transpose by a degenerate homing event with relaxed homology requirements, resulting in illegitimate recombination. Although no true transposition events by this pathway have yet been documented, evidence for transposition via reverse splicing into foreign RNAs is accumulating for group I and particularly for group II introns. The pathway involves reverse splicing of an excised intron into non-allelic RNA, followed by transfer into the genome at the heterologous site, most likely via a cDNA copy of the recombinant RNA (Fig. 2B).Group I Intron TranspositionAlthough RNA-mediated group I intron transposition has not yet been observed in its entirety, partial reactions have been noted both in vitro and in vivo. Reverse splicing of the Tetrahymena thermophila group I intron has been demonstrated into both its natural target site and into foreign RNAs that contain short sequences homologous to the normal ligation junction(14.Roman J. Woodson S.A. RNA. 1995; 1: 478-490PubMed Google Scholar, 15.Woodson S.A. Cech T.R. Cell. 1989; 57: 335-345Abstract Full Text PDF PubMed Scopus (162) Google Scholar). The target sequence can be as small as 4 nucleotides so long as it can pair with an internal guide sequence in the intron to direct the integration reaction. Proteins that normally promote splicing of specific introns can also facilitate reverse splicing in vitro, as was demonstrated for the Neurospora LSU intron and its splicing effector CYT-18 protein(16.Mohr G. Lambowitz A.M. Nature. 1991; 354: 164-167Crossref PubMed Scopus (20) Google Scholar).The first demonstration of a partial reverse splicing reaction in vivo was with the Cr.LSU intron, a chloroplast group I intron from Chlamydomonas reinhardtii(17.Thompson A.J. Herrin D.L. J. Mol. Biol. 1994; 236: 455-468Crossref PubMed Scopus (22) Google Scholar). This intron has been shown to undergo the first step of reverse splicing into the cytoplasmic 5.8 S rRNA of its host in vivo and in vitro. Minor changes in the 5.8 S sequence would allow complete integration of the intron. Nevertheless, how cDNA synthesis would ensue for introns that do not encode their own RT activity remains unclear, although a role for trans-acting cellular RTs can be readily envisaged.The preferential occurrence of group I introns in rRNA and tRNA genes may reflect the abundance of these RNAs, which could provide copious targets for reverse splicing in vivo(17.Thompson A.J. Herrin D.L. J. Mol. Biol. 1994; 236: 455-468Crossref PubMed Scopus (22) Google Scholar). Although integration into these targets may be a common phenomenon(14.Roman J. Woodson S.A. RNA. 1995; 1: 478-490PubMed Google Scholar), maintenance of stable RNA function and trapping of the intron insertion event through capture by the genome (e.g. via a cDNA intermediate) are likely to be rare. Such infrequent events would be most likely to occur with abundant RNAs.Group II Intron TranspositionThe discovery of twintrons in the chloroplast DNA of Euglena gracilis was the first indication that group II introns can transpose to other genomic sites (18.Copertino D.W. Hallick R.B. Trends Biochem. Sci. 1993; 18: 467-471Abstract Full Text PDF PubMed Scopus (89) Google Scholar). In the simplest twintrons, the internal intron disrupts splicing of the external intron so that the splicing pathway is ordered. The formation of twintrons has been explained by an intron reverse splicing into another intron by the pathway proposed in Fig. 2B. This pathway is further supported by the demonstration of the reversal of the self-splicing reaction in vitro and by integration of a group II intron into foreign RNA and DNA(19.Augustin S. Muller M.W. Schweyen R.J. Nature. 1990; 343: 383-386Crossref PubMed Scopus (86) Google Scholar, 20.Morl M. Niemer I. Schmelzer C. Cell. 1992; 70: 803-810Abstract Full Text PDF PubMed Scopus (55) Google Scholar, 21.Morl M. Schmelzer C. Cell. 1990; 60: 629-636Abstract Full Text PDF PubMed Scopus (75) Google Scholar).Several recent studies aimed at understanding site-specific deletions of fungal mtDNAs led to the discovery that group II introns that encode RT-like proteins can transpose to ectopic sites in mtDNAs. For example, intron 1 of the COX1 gene of yeast mtDNA, a group II intron that can also carry out site-specific homing(8.Lazowska J. Meunier B. Macadre C. EMBO J. 1994; 13: 4963-4972Crossref PubMed Scopus (60) Google Scholar), has been inferred to reverse splice into several sites in a group I intron of the COX1 gene, aI5β, to form twintrons with an internal group II intron and an external group I intron(22.Mueller M.W. Allmaier M. Eskes R. Schweyen R.J. Nature. 1993; 366: 174-176Crossref PubMed Scopus (85) Google Scholar). A similar intron in Podospora mtDNA was found to transpose to a site in mtDNA near a tRNA gene(23.Sellem C.H. Lecellier G. Belcour L. Nature. 1993; 366: 176-178Crossref PubMed Scopus (98) Google Scholar), whereas the group II intron of Schizosaccharomyces pombe mtDNA was found to transpose to multiple sites(24.Schmidt W.R. Schweyen R.J. Wolf K. Mueller M.W. J. Mol. Biol. 1994; 243: 157-166Crossref PubMed Scopus (30) Google Scholar). In each case, an RNA-mediated event involving RT was inferred (Fig. 2B). After ectopic insertion, the genome would contain two copies of the intron, so that homologous recombination would result in the deletion of one intron copy plus sequences between them. This type of deletion event could explain the circular αSen DNA, which contains a group II intron and is involved in the senescence phenomenon in Podospora(23.Sellem C.H. Lecellier G. Belcour L. Nature. 1993; 366: 176-178Crossref PubMed Scopus (98) Google Scholar) (reviewed in (2.Lambowitz A.M. Belfort M. Annu. Rev. Biochem. 1993; 62: 587-622Crossref PubMed Scopus (534) Google Scholar)).Although no cDNA intermediate for these transposition events has yet been demonstrated, other studies in yeast mitochondria support the inference that such cDNAs can be made. For example, the RT activity overproduced in a mutant of the aI2 intron deleted for a catalytic domain (domain 5) is much less specific for aI2 and, instead, uses other mitochondrial RNAs as a template(9.Moran J.V. Zimmerly S. Eskes R. Kennell J.C. Lambowitz A.M. Butow R.A. Perlman P.S. Mol. Cell. Biol. 1995; 15: 2828-2838Crossref PubMed Scopus (103) Google Scholar). Furthermore, reverse transcription and cDNA synthesis are strongly implicated in the intron loss phenomenon (Fig. 2C), which has been reported for both group I and group II introns of the COB and COX1 genes of yeast mtDNA. These events are detected among revertants of some intron mutants and always involve loss of the mutant intron. Frequently adjacent (and unmutated) introns are lost simultaneously (for example, see (25.Levra-Juillet E. Boulet A. Seraphin B. Simon M. Faye G. Mol. Gen. Genet. 1989; 217: 168-171Crossref PubMed Scopus (47) Google Scholar)). Since the exons separating the lost introns are retained, it was proposed that spliced mRNAs are reverse transcribed and recombined into mtDNA, resulting in loss of the introns. The RT-encoding group II introns are the likely source of the RT activity since strains lacking both aI1 and aI2 do not undergo intron loss(25.Levra-Juillet E. Boulet A. Seraphin B. Simon M. Faye G. Mol. Gen. Genet. 1989; 217: 168-171Crossref PubMed Scopus (47) Google Scholar).Proteins That Promote Intron MobilityThe proteins encoded by the group I introns comprise four families of endonucleases, whereas the group II intron proteins form one fairly homogenous class of RT-like proteins. Interestingly, a relationship has been established between one of the group I intron endonuclease families and a Zn2+ finger-like motif in some group II intron-encoded proteins(26.Gorbalenya A.E. Protein Sci. 1994; 3: 1117-1120Crossref PubMed Scopus (101) Google Scholar, 27.Shub D.A. Goodrich-Blair H. Eddy S.R. Trends Biochem. Sci. 1994; 19: 402-404Abstract Full Text PDF PubMed Scopus (132) Google Scholar). The recently discovered group II intron endonuclease activity is likely to be associated with this motif(10.Zimmerly S. Guo H. Perlman P.S. Lambowitz A.M. Cell. 1995; 82: 1-20Abstract Full Text PDF PubMed Scopus (231) Google Scholar). Additionally, several of the proteins encoded by both group I and group II introns have maturase function to promote splicing of their cognate intron (reviewed in Refs. 2 and 28). The reading frames of the group I and group II introns may occur in freestanding form within the intron or in-frame with the upstream exon (reviewed in (28.Mueller J.E. Bryk M. Loizos N. Belfort M. Linn S.M. Lloyd R.S. Roberts R.J. Nucleases. 2nd Ed. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY1993: 111-143Google Scholar)). In the latter case the precursor protein appears to be processed proteolytically to release the active intron-encoded protein(2.Lambowitz A.M. Belfort M. Annu. Rev. Biochem. 1993; 62: 587-622Crossref PubMed Scopus (534) Google Scholar, 28.Mueller J.E. Bryk M. Loizos N. Belfort M. Linn S.M. Lloyd R.S. Roberts R.J. Nucleases. 2nd Ed. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY1993: 111-143Google Scholar, 29.Guo W.W. Moran J.V. Hoffman P. Henke R.M. Butow R.A. Perlman P.S. J. Biol. Chem. 1995; 270: 15563-15570Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar, 30.Wernette C.M. Saldanha R. Perlman P.S. Butow R.A. J. Biol. Chem. 1990; 265: 18976-18982Abstract Full Text PDF PubMed Google Scholar).The LAGLIDADG ProteinsThe LAGLIDADG consensus sequence, which occurs as repeats (P1 and P2) flanking a region of 110 ± 40 amino acids(31.Hensgens L.A.M. Bonen L. da Haan M. van der Horst G. Grivell L.A. Cell. 1983; 32: 379-389Abstract Full Text PDF PubMed Scopus (156) Google Scholar), is present in the majority of homing endonucleases (2.Lambowitz A.M. Belfort M. Annu. Rev. Biochem. 1993; 62: 587-622Crossref PubMed Scopus (534) Google Scholar, 28.Mueller J.E. Bryk M. Loizos N. Belfort M. Linn S.M. Lloyd R.S. Roberts R.J. Nucleases. 2nd Ed. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY1993: 111-143Google Scholar) (Fig. 3A). The motif is phylogenetically in three and is present in endonucleases of group I introns, introns and as as in four known group I intron-encoded The number of highly conserved among the endonucleases is limited consensus in Fig. 3A). However, DNA cleavage activity has been to the conserved of both and J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar, R.A. 1992; 20: PubMed Scopus Google Scholar). This is since some endonucleases have only (Fig. 3A). mutants of the two conserved of maturase that motif in maturase function as R.M. Butow R.A. Perlman P.S. EMBO J. 1995; 14: PubMed Scopus (47) Google Scholar). The protein encoded by of yeast mtDNA has both endonuclease and maturase P.S. Butow R.A. Science. 1989; PubMed Scopus Google Scholar). of the conserved of of the protein endonuclease activity, but maturase R.M. Butow R.A. Perlman P.S. EMBO J. 1995; 14: PubMed Scopus (47) Google Scholar). these that is more important for endonuclease activity has a role in maturase endonuclease in intron-encoded and proteins. in in from the of the intron reading the contains the number of amino to the of the when to in the In each case, the amino for which the motif is are indicated in black the conserved amino are indicated the conserved in of in or or or in or depending on whether or of at a A, the LAGLIDADG The consensus was from the depicted which proteins with demonstrated endonuclease and endonucleases of The is from (28.Mueller J.E. Bryk M. Loizos N. Belfort M. Linn S.M. Lloyd R.S. Roberts R.J. Nucleases. 2nd Ed. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY1993: 111-143Google Scholar), which endonucleases J. R.A. 1994; PubMed Scopus Google Scholar) and M. M.W. Cell. 1993; Full Text PDF PubMed Scopus Google Scholar, B. M. Sci. S. A. 1994; PubMed Scopus (32) Google Scholar, M. Sci. S. A. 1992; PubMed Scopus (75) Google Scholar, G.-L. F. Macadre C. Slonimski P.P. Lazowska J. J. Mol. Biol. 1991; PubMed Scopus Google Scholar). B, the was by the of the sequence and of sequences E. was from the three proteins with demonstrated endonuclease to a with each sequence an C, the The consensus motif was from 40 proteins as A.E. Protein Sci. 1994; 3: 1117-1120Crossref PubMed Scopus (101) Google Scholar, 27.Shub D.A. Goodrich-Blair H. Eddy S.R. Trends Biochem. Sci. 1994; 19: 402-404Abstract Full Text PDF PubMed Scopus (132) Google Scholar). the group II and those proteins with demonstrated endonuclease activity are and endonuclease from group I intron endonucleases and J.E. Bryk M. Loizos N. Belfort M. Linn S.M. Lloyd R.S. Roberts R.J. Nucleases. 2nd Ed. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY1993: 111-143Google Scholar), group II intron from and J. F. Nature. 1993; PubMed Scopus Google Scholar), chloroplast group II intron from the and the S. mitochondrial group II protein from the aI2 intron The of a of a Zn2+ upstream of the motif and of the motif are the is from S. 1993; PubMed Scopus Google Large Image Figure ViewerDownload Hi-res image Download ProteinsThe two of the motif are by amino G. S. J. Mol. Biol. PubMed Scopus Google Scholar, F. B. Cell. 1986; Full Text PDF PubMed Scopus Google Scholar) and occur upstream of a conserved sequence of amino (Fig. This is the most common consensus sequence found in intron-encoded proteins (reviewed in (2.Lambowitz A.M. Belfort M. Annu. Rev. Biochem. 1993; 62: 587-622Crossref PubMed Scopus (534) Google Scholar) and (28.Mueller J.E. Bryk M. Loizos N. Belfort M. Linn S.M. Lloyd R.S. Roberts R.J. Nucleases. 2nd Ed. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY1993: 111-143Google Scholar)). occurs in both and group I intron-encoded endonucleases of phage T4 and is in group I intron proteins of fungal Unlike the LAGLIDADG no studies have yet been reported with the motif and has the motif yet been associated with either or Proteins and the Zn2+ proteins contain a consensus sequence amino with two highly conserved of contained within a less conserved domain of amino A.E. Protein Sci. 1994; 3: 1117-1120Crossref PubMed Scopus (101) Google Scholar, 27.Shub D.A. Goodrich-Blair H. Eddy S.R. Trends Biochem. Sci. 1994; 19: 402-404Abstract Full Text PDF PubMed Scopus (132) Google Scholar) (Fig. The motif occurs in group I intron endonucleases of both phage and and in group I intron proteins of In to its in a number of other endonucleases, and the motif appears in a Zn2+ finger-like domain of group II intron The occurrence of this endonuclease motif in group I and group II intron proteins likely the of endonuclease function in both intron mobility with a role for this domain as an endonuclease involved in the a mutant in the domain of the aI2 intron protein endonuclease activity and J.V. Zimmerly S. Eskes R. Kennell J.C. Lambowitz A.M. Butow R.A. Perlman P.S. Mol. Cell. Biol. 1995; 15: 2828-2838Crossref PubMed Scopus (103) Google Scholar, 10.Zimmerly S. Guo H. Perlman P.S. Lambowitz A.M. Cell. 1995; 82: 1-20Abstract Full Text PDF PubMed Scopus (231) Google Scholar).The ProteinsThe is a conserved region in three homing endonucleases of and an S. 1993;