Evolution of aminobenzoate synthases: nucleotide sequences of Salmonella typhimurium and Klebsiella aerogenes pabB.

p-Aminobenzoate synthase (PS) and anthranilate synthase (AS) are structurally related enzymes that catalyze similar reactions and produce similar products, para- and ortho-aminobenzoate (anthranilate). Each enzyme is composed of two non-identical subunits: a glutamine amidotransferase subunit (CoII) and a subunit that produces an aminobenzoate product (CoI). Nucleotide sequence comparisons of the Escherichia coli genes encoding each of the subunits suggest a common evolutionary origin for both subunits of the enzyme complexes. We report here the nucleotide sequences of the pabB genes that encode Salmonella typhimurium and Klebsiella aerogenes PS CoI. Comparative sequence information suggests that pabB is encoded as the first gene in a multicistronic transcript. Comparison of deduced amino acid sequences of PS CoI genes indicates that the majority of sequence identity occurs in the C-terminal two-thirds of the proteins. Similarly, identities in an alignment of eight PS and AS CoI sequences are confined to the C-terminal segments of the proteins. Secondary-structure predictions for the nine sequences suggest considerable similarity in the folding of the C-terminal portions of the aminobenzoate synthases.