Phototaxis in a wild isolate of the cyanobacterium <i>Synechococcus elongatus</i>.

Abstract

Many cyanobacteria, which use light as an energy source via photosynthesis, have evolved the ability to guide their movement toward or away from a light source. This process, termed "phototaxis," enables organisms to localize in optimal light environments for improved growth and fitness. Mechanisms of phototaxis have been studied in the coccoid cyanobacterium <i>Synechocystis</i> sp. strain PCC 6803, but the rod-shaped <i>Synechococcus elongatus</i> PCC 7942, studied for circadian rhythms and metabolic engineering, has no phototactic motility. In this study we report a recent environmental isolate of <i>S. elongatus</i>, the strain UTEX 3055, whose genome is 98.5% identical to that of PCC 7942 but which is motile and phototactic. A six-gene operon encoding chemotaxis-like proteins was confirmed to be involved in phototaxis. Environmental light signals are perceived by a cyanobacteriochrome, PixJ_Se (Synpcc7942_0858), which carries five GAF domains that are responsive to blue/green light and resemble those of PixJ from <i>Synechocystis</i> Plate-based phototaxis assays indicate that UTEX 3055 uses PixJ_Se to sense blue and green light. Mutation of conserved functional cysteine residues in different GAF domains indicates that PixJ_Se controls both positive and negative phototaxis, in contrast to the multiple proteins that are employed for implementing bidirectional phototaxis in <i>Synechocystis</i>.