INDUCTION OF THE GAP-PGK OPERON ENCODING GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASE AND 3-PHOSPHOGLYCERATE KINASE OF XANTHOBACTER-FLAVUS REQUIRES THE LYSR-TYPE TRANSCRIPTIONAL ACTIVATOR CBBR

In a previous study, a gene (pgk) encoding phosphoglycerate kinase was isolated from a genomic labrid of Xanthobacter flavus. Although this gene is essential for autotrophic growth, it is not located within the cbb operon encoding other Calvin cycle enzymes. An analysis of the nu cleotide sequence upstream from pgk showed the presence of a gene enco ding glyceraldehyde-3-phosphate dehydrogenase and the 3' end of an ope n reading frame encoding a protein which is 50% identical to transketo lase encoded by cbbT of X. flavus. Gene fusions between pgk and lacZ d emonstrated that the gap and pgk genes are organized in an operon. Ind uction of the Calvin cycle in heterotrophically growing cells resulted in a sixfold increase in phosphagrlycerate kinase activity in paralle l with the appearance of ribulosebisphosphate carboxylase activity. Th is superinduction of phosphoglycerate kinase did not occur in an X. fl avus strain in which cbbR, encoding the transcriptional activator of t he cbb operon, was disrupted. The failure to superinduce the gap-pgk o peron is not caused by the absence of a functional Calvin cycle, since the expression of this operon in an X. flavus strain with a defective ribulosebisphosphate carboxylase enzyme was the same as the expressio n in the wild type. It is therefore concluded that the expression of b oth the cbb and gap-pgk operons is controlled by CbbR.