The nucleotide sequence of a 1:1 kb l-III fragment downstream of the malate dehydrogenase () gene of revealed the presence of an ORF and an incomplete ORF lacking its NH-terminal portion, in the opposite orientation to that of the gene. These two genes overlapped with each other, sharing two base pairs, suggesting that these genes are co-transcribed in a single mRNA. One ORF (termed ) encoded a protein of 154 amino acids showing significant amino acid sequence similarity to purine phosphoribosyltransferases, such as xanthine-guanine phosphoribosyltransferase of and human hypoxanthine phosphoribosyltransferase. Cloning and sequencing of the upstream region of the gene, together with sequence comparison of the gene product encoded by the region upstream of , suggested that the upstream ORF encoded two in-frame overlapping aspartokinase genes, , encoding the β-subunit of 405 amino acids, and , encoding the β-subunit of 161 amino acids, which was part of the 3′ portion of . Consistent with the sequence data, the and the genes conferred the heat-stable enzyme activities of aspartokinase and phosphoribosyltransferase, respectively, on . Preliminary characterization of these enzymes produced in is described.


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