The model cyanobacterium Synechocystis sp. PCC 6803 is used extensively in the studies of oxygenic photosynthesis as well as genetic engineering. The ability to incorporate exogenous DNA into its genome through homologous recombination makes it a potential biosystem that can be manipulated to produce valuable compounds. Synechocystis sp. PCC 6803 possesses an advanced DNA recombination and repair mechanism that enables the survival of the bacterium in various environments. This mechanism serves as a double edge sword as it improves the plasticity of the cyanobacterium; on the other hand, it also promotes genomic instability. This study investigated the role of the genes sll1354 (recJ), sll1165 (mutS1), and sll1520 (recN) in genomic instability. A PS II mutant strain showed signs of genomic instability where it grew photoautotrophically in pH 7.5 medium, which it is incapable of growing initially. When the constructs of interrupted genes of interest were introduced into the strain, the mutants' ability to grow photoautotrophically in pH 7.5 medium was compared to the control. The interruption of sll1354 and sll1520 was found to show a minimal effect on the growth rate reversion, whereas disruption on sll1165 was found to restored photoautotrophic growth in pH7.5 medium. This preliminary data in this study suggested that the gene sll1165 may be one of the genes that is involved in the DNA recombination and repair mechanism in Synechocystis sp. PCC 6803, which, in turn, may plays a role in cyanobacterial genomic instability
