Two partially homologous adjacent light-inducible maize chloroplast genes encoding polypeptides of the P700 chlorophyll a-protein complex of photosystem I

Abstract

The maize chloroplast chromosome contains two light-inducible genes, ps1A1 and ps1A2, that code for 45% homologous polypeptides of 83.2 and 82.5 kDa designated A1 and A2, respectively. Two types of immunochemical evidence show that the upstream gene, ps1A1, codes for a P700 chlorophyll a-protein at the reaction center of photosystem I of the photosynthetic apparatus. Antibodies against a synthetic peptide with a sequence deduced from the DNA sequence of an unconserved segment of A1 react with polypeptides of P700 chlorophyll-protein (CPI) complexes of maize and pea photosystem I; antibodies prepared against barley CPI immunoprecipitate products of in vitro transcription and translation directed by cloned chloroplast DNA containing this gene. The extensive homology between maize polypeptides A1 and A2 suggests that both may be components of CPI, although CPI has been generally considered to be comprised of two molecules of a single protein of only 66 to 70 kDa. The hexapeptide Asp-Pro-Thr-Thr-Arg-Arg in A2 is also present in another chlorophyll protein, the P680 chlorophyll a-protein of photosystem II, and is partially duplicated in A1. The number and locations of histidyl residues, which have been suggested to serve in chlorophyll binding, are highly conserved between A1 and A2.