Photosynthetic mutants of the cyanobacteria Synechocystis sp. strains PCC 6714 and PCC 6803: sodium p-hydroxymercuribenzoate as a selective agent.

Among a wide range of potential selective agents examined, sodium p-hydroxymercuribenzoate successfully enriched for mutants of Synechocystis sp. strains PCC 6714 and 6803 defective in photosynthesis. When both photosystems I and II were operating, viability of wild-type cells decreased to between 5 X 10(-5) and 1 X 10(-6) after 5 h of incubation with 500 microM p-hydroxymercuribenzoate (strain 6714), and after 8 h with 200 microM (strain 6803). Between 0.1 and 0.5% of the survivors were stable mutants defective in different steps of photosynthesis. The compound was not mutagenic. It was less toxic to cells grown chemoheterotrophically in the dark or photoheterotrophically in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. p-Hydroxymercuribenzoate therefore killed only cells which were performing photosynthesis at high rates, thereby specifically selecting for mutants deficient in this process.