A cyanobacterial gene family coding for single-helix proteins resembling part of the light-harvesting proteins from higher plants

In the cyanobacterium Synechocystis sp. PCC 6803 five genes were identified with significant sequence similarity to regions of members of the eukaryot ic chlorophyll nib binding gene family (Cab family) and to hliA, a gene cod ing for a small high-light-induced protein in Synechococcus sp. PCC 7942. F our of these five genes are 174-213 bp in length and code for small protein s predicted to have a single transmembrane helix. The fifth Cab-like gene i n Synechocystis sp. PCC 6803 is much longer and codes for a protein of whic h the N-terminal 80% resemble ferrochelatase but the C-terminal domain has similarity to Cab regions. The small genes were expressed preferentially in the absence of photosystem I, but gene expression was not significantly en hanced at moderately high light intensity. Therefore they were not designat ed as hli (high-light-induced) as was done for the Synechococcus sp. PCC 79 42 homolog. Instead, the genes have been named sep, as the corresponding po lypeptides of Synechocystis sp. PCC 6803 are small Cab-like proteins (SCP). The scpA gene, which codes for ferrochelatase with a C-terminal Cab-like e xtension, was interrupted by the insertion of a kanamycin-resistance casset te between the ferrochelatase and Cab-like gene domains. In the PS I-less b ackground, interruption of scpA was found to lead to increased tolerance to high light intensity and to the requirement of a slightly higher light int ensity to drive photosystem II electron transfer, suggestive of decreased l ight-harvesting efficiency in the absence of the C-terminal extension of Sc pA. Immunodetection of ScpC and ScpD indicated that either or both accumula ted in PS I-less strains. These proteins were also detected in bands of mor e than 45 kDa on denaturing gels, raising the possibility that they may occ ur as stable oligomers. The SCPs represent a new group of cyanobacterial pr oteins that, in view of their primary structure and response to deletion of photosystem I, are likely to be involved in transient pigment binding.