No abstract

The gene for the FeS protein of the Rhodopseudomonas sphaeroides b/c1 complex was identified by means of cross‐hybridization with a segment of the gene encoding the corresponding FeS protein of Neurospora crassa. Plasmids (pRSF1‐14) containing the cross‐hybridizing region, covering in total 13.5 kb of chromosomal DNA, were expressed in vitro in a homologous system. One RSF plasmid directed the synthesis of all three main polypeptides of the R. sphaeroides b/c1 complex: the FeS protein, cytochrome b and cytochrome c1. The FeS protein and cytochrome c1 were apparently synthesized as precursor forms. None of the pRSF plasmids directed the synthesis of the 10‐kd polypeptide found in b/c1 complex preparations. Partial sequencing of the cloned region was performed. Several sites of strong homology between R. sphaeroides and eukaryotic polypeptides of the b/c1 complex were identified. The genes encode the three b/c1 polypeptides in the order: (5′) FeS protein, cytochrome b, cytochrome c1. The three genes are transcribed to give a polycistronic mRNA of 2.9 kb. This transcriptional unit has been designated the fbc operon; its coding capacity corresponds to the size of the polycistronic mRNA assuming that only the genes for the FeS protein (fbcF), cytochrome b (fbcB) and cytochrome c1 (fbcC) are present. This could indicate that these three subunits constitute the minimal catalytic unit of the b/c1 complex from photosynthetic membranes.

Mitochondrially synthesized polypeptides of human, monkey, and mouse cells were compared using SDS-polyacrylamide gel electrophoresis (SDS-PAGE). A single molecular weight variant, the major interspecific variant (MIV), was identified in human cells as compared to monkey and mouse. The peptide maps of MIV were compared between the three species using a two-dimensional proteolytic digest (2D-PD) gel system. A number of conserved peptides were found, indicating that the MIVs have a common function. Other MIV peptides were species specific. These results confirm the conserved nature of mitochondrial polypeptides and demonstrate the utility of 2D-PD gels in testing for protein alleles and detecting subtle protein variants.