Covalent modification of a 13 kDa soluble-phase protein occurs during adaptation of cells of the cyanobacterium Synechococcus 6301 (mutant AN 112) to light specifically absorbed by photosystem II. This adaptation is accompanied by functional changes indicative of altered excitation energy distribution between the photosystems. The 13 kDa protein is identified by solid-phase N-terminal sequencing as a protein related to PII, the glnB gene product of E. coli. In E. coli, the PII protein undergoes uridylylation and acts as a regulator of glutamine synthetase at both the post-translational and transcriptional levels. The implications of modification of a transcriptional regulator by photosynthetic electron transport are discussed.
