Deletion of the psbG1 gene of the cyanobacterium Synechocystis sp. PCC6803 leads to the activation of the cryptic psbG2 gene

The genes psbG1 and psbG2 in the cyanobacterium Synechocystis sp. PCC6803 are homologous. The psbG1 gene is located on the chromosome and is part of the ndhC--psbG1--ORF157 operon, while psbG2 is located on a plasmid and is not flanked by equivalent ndhC or ORF157 genes. Mutants in which psbG1 is deleted grow well under autotrophic conditions, while their growth is impeded in mixotrophic medium. These results argue against a functional role for psbG1 in photosynthesis, i.e. photosystem II, and are more compatible with a function in respiration. The psbG2 gene is not transcribed in wild-type cells, but in psbG1 mutants the insertion of DNA sequences in close proximity to the psbG2 reading frame has led to transcriptional activation of psbG2. Thus, psbG2 represents an example of a cryptic gene, similar to those found in other bacteria.