The expression of the and operons of encoding enzymes of the Calvin cycle is regulated by the transcriptional regulator CbbR. In order to identify other genes involved in the regulation of these operons, a mutant was isolated with a lowered activity of a fusion between the promoter of the operon and the reporter gene This mutant was unable to grow autotrophically and had a reduced growth rate of medium supplemented with gluconate or succinate. The regulation of the operon in the mutant was indistinguishable from the wild-type strain, but induction of the operon upon transition to autotrophic growth conditions was delayed. Complementation of the mutant with a genomic library of resulted in the isolation of a 1.1 kb l fragment which restored autotrophic growth of the mutant. One open reading frame (ORF) was present on the l fragment, which could encode a protein highly similar to triosephosphate isomerase proteins from other bacteria. Cell extracts of the mutant grown under glycolytic or gluconeogenic conditions had severely reduced triosephosphate isomerase activities. The ORF was therefore identified as encoding triosephosphate isomerase. The gene is not linked to the previously identified operons encoding Calvin cycle enzymes and therefore represents a third transcriptional unit required for autotrophic metabolism.


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