A deletion mutation in the spacing within the psaA core promoter enhances transcription in a cyanobacterium Synechocystis sp. PCC 6803.

Transcriptional regulation of PSI reaction center psaA is one of the important physiological responses to changing environments. We previously reported that the Rrf2-type transcriptional regulator Slr0846 activates transcription of psaA in Synechocystis sp. PCC 6803. In the Δslr0846 mutant, transcripts from two promoters, P1 and P2, were downshifted and, as a result, a lower Chl content and slower growth were observed. Here, we report spontaneous suppressors which recovered Chl accumulation and photoautotrophic growth. Sequencing of the whole promoter region revealed in some suppressors the same single nucleotide deletion in a 9 bp G stretch (-21 to -29 from the transcriptional start point of P1), which is located between the -35 and -10 elements of the P1 core promoter (hereafter the -G mutation). The transcripts from P1 were higher in abundance in this pseudorevertant than in the Δslr0846 mutant. When the promoter was fused to a reporter gene, the -G mutation conferred ~4 times higher expression than the wild-type promoter. It has been shown that the P1 promoter activity of psaA is regulated by a high light regulatory element 1 just upstream of -35. The -G mutated P1 promoter still retained the high light response. Thus, the -G mutation enhanced the expression level of psaA without a loss of the response to the high light conditions. This is the first study of the spontaneous mutation of a spacer length of a promoter for expression in cyanobacteria.