Primary structure of the M subunit of the reaction center from Rhodopseudomonas sphaeroides

Abstract

The reaction center is a membrane-bound bacteriochlorophyll-protein complex that mediates the primary photochemical events in R. sphaeroides. The previously determined amino-terminal sequences of the 3 subunits of the reaction center protein were used to design synthetic mixed oligonucleotide probes for the structural genes encoding the subunits. One probe was used to isolate and clone a fragment of DNA from R. sphaeroides that contained the gene encoding the M subunit. The nucleotide sequence of this gene was determined by the dideoxy method. Several tryptic and chymotryptic peptides from the M protein were isolated and subjected to sequence analysis and the sequence of the carboxyl terminus was determined. Together with the amino-terminal sequence, the data establish the primary structure of the M protein. The distribution of hydrophobic residues in the amino acid sequence suggests the presence of 5 membrane-spanning segments. A significant homology was found between the amino acid sequence of the M subunit and a thylakoid membrane protein (MW 32,000) from spinach that has been implicated in herbicide and quinone binding.