Phenotypic complementation of high CO₂-requiring mutants of the cyanobacterium Synechococcus sp. strain PCC 7942 by inosine 5′-monophosphate

The high CO₂-requiring mutants of Synechococcus PCC 7942, D4 and R14, were obtained by deletion or inactivation (respectively) of an open reading frame immediately downstream of rbc (the operon encoding the subunits of ribulose 1,5-bisphosphate carboxylase/oxygenase). These mutants exhibit photosynthetic characteristics similar to those of high CO₂-grown wild type, unlike other cyanobacterial high CO₂-requiring mutants, where the apparent photosynthetic affinity for inorganic carbon is approximately 2 orders of magnitude lower than that of the wild type. Sequence analysis and metabolic complementation of the mutants by inosine 5′-monophosphate identified this open reading frame as the cyanobacterial equivalent of purK, the eubacterial gene encoding subunit II of phosphoribosyl aminoimidazole carboxylase in the purine biosynthetic pathway. Exposure of high CO₂-grown Synechococcus to low CO₂ conditions led to the induction of transcription of purK. It is suggested that the high CO₂-requiring phenotype of these mutants resulted from the defect in purine biosynthesis after exposure to low CO₂. We also raise the possibility that the level of cellular purines is involved in the process of adaptation of cyanobacteria to low concentrations of CO₂.