MUTATION IN A NOVEL GENE REQUIRED FOR PHOTOMIXOTROPHIC GROWTH LEADS TO ENHANCED PHOTOAUTOTROPHIC GROWTH OF SYNECHOCYSTIS SP. PCC-6803

In the glucose-tolerant strain of Synechocystis sp. PCC 6803, we found two types of cells with distinct growth properties. Under photoautotr ophic conditions at any light intensity, one type gave larger colonies (designated WL) than the other (designated WS). Notably, the WL cells produced much larger colonies than the WS cells at higher light inten sity. In contrast, growth of the WL cells was severely suppressed unde r mixotrophic conditions with glucose and light, while the WS cells gr ew normally. A gene which could complement the WL phenotype was obtain ed from a wild-type genomic library. The gene, designated pmgA, coded for a 23 kDa polypeptide of 204 amino acid residues with no apparent h omology to known genes. In the WL genome, the base substitution of T f or C at position 193 of pmgA caused replacement of Leu with Phe at pos ition 65 of the product. The phenotype of pmgA disruption mutants was similar to that of the WL cells, indicating that the WS cells expresse d a functional pmgA product. By direct sequencing of polymerase chain reaction-amplified pmgA from genomic DNA, it was revealed as an exampl e of microevolution that WL had expelled WS from the photoautotrophic culture of wild-type in our laboratory for a year or so. Mixed culture in liquid also demonstrated that the WL cells increased gradually und er photoautotrophic conditions, while they decreased rapidly under pho tomixotrophic conditions. These results suggest that pmgA product is e ssential for photomixotrophic growth, whereas it represses photoautotr ophic growth. To our knowledge, the WL cells and pmgA-disrupted mutant s are the first in cyanobacteria, which shows much improved photosynth etic growth than wild-type especially at high light intensity.