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In the roots of barley and other cereals, hypoxia induces a set of five isozymes of L-lactate dehydrogenase LDH; (S)-lactate:NADH oxidoreductase, EC 1.1.1.27. Biochemical and genetic data indicate that the five LDH isozymes are tetramers that arise from random association of the products of two Ldh loci. To investigate this system, cDNA clones of LDH were isolated from a lambda gt11 cDNA library derived from hypoxically treated barley roots. The library was screened with antiserum raised against barley LDH purified approximately 3000-fold by an improved three-step procedure. Immunopositive clones were rescreened with a cDNA probe synthesized by the polymerase chain reaction using primers modeled from the amino acid sequences of two tryptic LDH peptides. Two types of LDH clones were found. Nucleotide sequence analysis of one representative insert of each type (respectively, 1305 and 1166 base pairs) revealed open reading frames encoding 10 peptide fragments of LDH. The 1305-base-pair insert included the entire coding region of a 356-residue LDH monomer. The nucleotide sequences of the two LDH cDNAs were 92% identical in the coding region, but highly divergent in the 3' noncoding region, and thus probably correspond to the two postulated Ldh loci. The deduced amino acid sequences of the two barley LDHs were 96% identical to each other and very similar to those from vertebrate and bacterial LDHs. RNA blot hybridization showed a single mRNA band of 1.5 kilobases whose level rose about 8-fold in roots during hypoxic induction, as did the level of translatable LDH message.
Hondred et al. (Sat,) studied this question.