Disruption of a gene encoding a novel thioredoxin-like protein alters the cyanobacterial photosynthetic apparatus.

A gene that may encode a novel protein disulfide oxidoreductase, designated txlA (thioredoxin-like), was isolated from the cyanobacterium Synechococcus sp. strain PCC7942. Interruption of txlA near the putative thioredoxin-like active site yielded cells that grew too poorly to be analyzed. In contrast, a disruption of txlA near the C terminus that left the thioredoxin-like domain intact yielded two different mutant phenotypes. One type, designated txlXb, exhibited a slightly reduced growth rate and an increased cellular content of apparently normal phycobilisomes. The cellular content of phycobilisomes also increased in in the other mutant strain, designated txlXg. However, txlXg also exhibited a proportionate increase in chlorophyll and other components of the photosynthetic apparatus and grew as fast as wild-type cells. Both the txlXb and txlXg phenotypes were stable. The differences between the two strains may result from a genetic polymorphism extant in the original cell population. Further investigation of txlA may provide new insights into mechanisms that regulate the structure and function of the cyanobacterial photosynthetic apparatus.