CLONING AND NUCLEIC-ACID SEQUENCE OF THE SALMONELLA-TYPHIMURIUM PNCB GENE AND STRUCTURE OF NICOTINATE PHOSPHORIBOSYLTRANSFERASE

The pncB gene of Salmonella typhimurium, encoding nicotinate phosphoribosyltransferase (NAPRTase), was cloned on a 4.7-kb Sau3A fragment. The gene contains a 1,200-bp open reading frame coding for a 400-residue protein. Amino acid sequencing of the amino-terminal and two interior peptides of the purified protein confirmed the deduced sequence and revealed that the amino-terminal methionine residue was removed, giving a 399-residue mature protein of M(r) 45,512. No signal sequence was observed in the predicted NAPRTase primary structure, suggesting that the enzyme is not periplasmic. The protein does not demonstrate clear sequence similarity to the other seven phosphoribosyltransferases of known primary structure and frustrates attempts to define a consensus 5-phosphoribosyl-1-pyrophosphate-bindig region. The NAPRTase reaction is ATP stimulated, and the protein contains a carboxy-terminal sequence diagnostic of an ATP-binding site. An inverted repeat of the sequence TAAACAA observed in the proposed promoter region of pncB is also present in the promoter of nadA, which, like pncB, is also regulated by the NadR (NadI) repressor. The sequence may thus define an NadR repressor-binding site.