Molecular cloning of the human and murine 2-amino-3-ketobutyrate coenzyme A ligase cDNAs

The conversion of l-threonine to glycine in both prokaryotes and eukaryotes takes place through a two-step biochemical pathway involving the enzymes l -threonine dehydrogenase (EC 1.1.1103) and 2-amino-3-ketobutyrate coenzyme A ligase (KBL; EC 2.3.1.29). The genes encoding these enzymes have been des cribed in prokaryotes but not in eukaryotes. We report the cloning of trans cripts for KBL, the second enzyme in the pathway, from human and murine lun g and a partial transcript from bovine liver. Two peptide sequences from th e purified bovine KBL protein, one from the N-terminus and the other from t he peptide containing the pyridoxal 5'-phosphate-binding lysine residue [To ng, H. & Davis, L. (1994) J. Biol. Chem. 269, 4057-4064], are identical wit h regions of the conceptual translation of the transcript obtained from bov ine liver. The partial transcript from bovine liver was very similar to the human transcript, being 91% and 92% identical at the nucleotide and amino- acid levels, respectively. The human and murine KBL transcripts are 1.5 kb long, with ORFs encoding proteins of 419 and 416 residues, respectively. Th e mouse protein has 90% identity with the human protein. The human transcri pt is strongly expressed in heart, brain, liver and pancreas compared with the lung. The N-termini of both human and mouse proteins have characteristi cs of mitochondrial import sequences. Both human and murine proteins have 5 4% identity with the well-characterised prokaryote KLB protein from Escheri chia coli. Database searches with the human cDNA sequence enabled us to ide ntify the human KBL gene on chromosome 22q12-13, consisting of nine exons o ver 9 kb, and a hypothetical Caenorhabditis elegans KLB gene on chromosome IV, consisting of five exons over 2 kb.