SEQUENCE, EXPRESSION, AND FUNCTION OF THE GENE FOR THE NONPHOSPHORYLATING, NADP-DEPENDENT GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASE OF STREPTOCOCCUS-MUTANS

We report the sequencing of a 2,019-bp region of the Streptococcus mut ans NG5 genome which contains a 1,428-bp open reading frame (ORF) whos e putative translation product had 50% identity to the amino acid sequ ences of the nonphosphorylating, NADP-dependent glyceraldehyde-3-phosp hate dehydrogenases (GAPN) from maize and pea. This ORF is located app roximately 200 bp downstream of the ptsI gene coding for enzyme I of t he phosphoenolpyruvate:sugar phosphotransferase transport system. Muta nt BCH150, in which the putative gapN gene had been inactivated, lacke d GAPN activity that was present in the wild-type strain, thus positiv ely identifying the ORF as the S. mutans gapN gene. Another strain of S. mutans, DC10, which contains an insertionally inactivated ptsI gene , still possessed GAPN activity, as did S. salivarius ATCC 25975, whic h contains an insertion element between the ptsI and gapN genes. Since the wild-type S. mutans NG5 lacks both glucose-6-phosphate dehydrogen ase and NADH:NADP oxidoreductase activities, the NADP-dependent glycer aldehyde-3-phosphate dehydrogenase is important as a means of generati ng NADPH for biosynthetic reactions.