Primary structure of the reaction center from XXXX .

@entity1472 @entity11704 @entity535 @entity377 @entity1473
The reaction center is a pigment-protein complex that mediates the initial photochemical steps of photosynthesis. The @entity1472 -terminal sequences of the L, M, and H subunits and the @entity377 and derived @entity535 sequences of the L and M structural genes from @entity11704 have previously been determined. We report here the sequence of the H subunit, completing the primary structure determination of the reaction center from @entity11704 . The @entity377 sequence of the gene encoding the H subunit was determined by the dideoxy method after subcloning fragments into single-stranded M13 phage vectors. This information was used to derive the @entity535 sequence of the corresponding polypeptide. The termini of the primary structure of the H subunit were established by means of the @entity1472 and @entity1473 terminal sequences of the polypeptide. The data showed that the H subunit is composed of 260 residues, corresponding to a molecular weight of 28,003. A molecular weight of 100,858 for the reaction center was calculated from the primary structures of the subunits and the cofactors. Examination of the genes encoding the reaction center shows that the codon usage is strongly biased towards codons ending in G and C. Hydropathy analysis of the H subunit sequence reveals one stretch of hydrophobic residues near the @entity1472 terminus; the L and M subunits contain five such stretches. From a comparison of the sequences of homologous proteins found in bacterial reaction centers and photosystem II of plants, an evolutionary tree was constructed. The analysis of evolutionary relationships showed that the L and M subunits of reaction centers and the D1 and D2 proteins of photosystem II are descended from a common ancestor, and that the rate of change in these proteins was much higher in the first billion years after the divergence of the reaction center and photosystem II than in the subsequent billion years represented by the divergence of the species containing these proteins.
