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.

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RESUMO

In a previous study, a gene (pgk) encoding phosphoglycerate kinase was isolated from a genomic library 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 nucleotide sequence upstream from pgk showed the presence of a gene encoding glyceraldehyde-3-phosphate dehydrogenase and the 3' end of an open reading frame encoding a protein which is 50% identical to transketolase encoded by cbbT of X. flavus. Gene fusions between pgk and lacZ demonstrated that the gap and pgk genes are organized in an operon. Induction of the Calvin cycle in heterotrophically growing cells resulted in a sixfold increase in phosphoglycerate kinase activity in parallel with the appearance of ribulosebisphosphate carboxylase activity. This superinduction of phosphoglycerate kinase did not occur in an X. flavus strain in which cbbR, encoding the transcriptional activator of the cbb operon, was disrupted. The failure to superinduce the gap-pgk operon 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 expression in the wild type. It is therefore concluded that the expression of both the cbb and gap-pgk operons is controlled by CbbR.

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