Sll0939 is induced by Slr0967 in the cyanobacterium Synechocystis sp. PCC6803 and is essential for growth under various stress conditions.

In this study, the genes expressed in response to low pH stress were identified in the unicellular cyanobacterium Synechocystis sp. PCC 6803 using DNA microarrays. The expression of slr0967 and sll0939 constantly increased throughout 4-h acid stress conditions. Overexpression of these two genes under the control of the trc promoter induced the cells to become tolerant to acid stress. The Δslr0967 and Δsll0939 mutant cells exhibited sensitivity to osmotic and salt stress, whereas the trc mutants of these genes exhibited tolerance to these types of stress. Microarray analysis of the Δslr0967 mutant under acid stress conditions showed that expression of the high light-inducible protein ssr2595 (HliB) and the two-component response regulator slr1214 (rre15) were out of regulation due to gene inactivation, whereas they were upregulated by acid stress in the wild-type cells. Microarray analysis and real-time quantitative reverse transcription-polymerase chain reaction analysis showed that the expression of sll0939 was significantly repressed in the slr0967 deletion mutant. These results suggest that sll0939 is directly involved in the low pH tolerance of Synechocystis sp. PCC 6803 and that slr0967 may be essential for the induction of acid stress-responsive genes.