In the cyanobacterium Synechocystis sp. PCC 6803 five genes were identified with significant sequence similarity to regions of members of the eukaryotic chlorophyll a/b binding gene family (Cab family) and to hliA, a gene coding for a small high-light-induced protein in Synechococcus sp. PCC 7942. Four of these five genes are 174-213 bp in length and code for small proteins predicted to have a single transmembrane helix. The fifth Cab-like gene in Synechocystis sp. PCC 6803 is much longer and codes for a protein of which 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 enhanced at moderately high light intensity. Therefore they were not designated as hli (high-light-induced) as was done for the Synechococcus sp. PCC 7942 homolog. Instead, the genes have been named scp, as the corresponding polypeptides of Synechocystis sp. PCC 6803 are small Cab-like proteins (SCP). The scpA gene, which codes for ferrochelatase with a C-terminal Cab-like extension, was interrupted by the insertion of a kanamycin-resistance cassette between the ferrochelatase and Cab-like gene domains. In the PS I-less background, interruption of scpA was found to lead to increased tolerance to high light intensity and to the requirement of a slightly higher light intensity to drive photosystem II electron transfer, suggestive of decreased light-harvesting efficiency in the absence of the C-terminal extension of ScpA. Immunodetection of ScpC and ScpD indicated that either or both accumulated in PS I-less strains. These proteins were also detected in bands of more than 45 kDa on denaturing gels, raising the possibility that they may occur as stable oligomers. The SCPs represent a new group of cyanobacterial proteins that, in view of their primary structure and response to deletion of photosystem I, are likely to be involved in transient pigment binding.