Deletion of cytochrome c oxidase genes from the cyanobacterium Synechocystis sp. PCC6803: Evidence for alternative respiratory pathways.

An oligonucleotide directed against a highly conserved region of aa3-type cytochrome c oxidases was used to clone the cox genes from the cyanobacterium Synechocystis sp. PCC6803. Several overlapping clones were obtained that contained the coxB, coxA, and coxC genes, transcribed in the same direction in that order, coding for subunits II, I, and III, respectively. The deduced protein sequences of the three subunits showed high sequence similarity with the corresponding subunits of all known aa3-type cytochrome c oxidases. A 1.94-kb HindII fragment containing most of coxA and about half of coxC was deleted and replaced by a cassette coding for kanamycin resistance. Mutant cells that were homozygous for the deleted cox locus were obtained. They were viable under photoautotrophic and photoheterotrophic conditions, but contained no cytochrome c oxidase activity. Nevertheless, these mutant cells showed almost normal respiration, defined as cyanide-inhibitable O2 uptake by whole cells in the dark. It is concluded, therefore, that aa3-type cytochrome c oxidase is not the only terminal respiratory oxidase in Synechocystis sp. PCC6803.