The reaction center complex and photosynthesis genes in the green filamentous bacterium lacking chlorosomes, Roseiflexus castenholzii

Abstract

A new filamentous photosynthetic bacterium was recently isolated from a Japanese hot spring. The bacterium, 'Roseiflexus castenholzii' belongs to the group of green filamentous bacteria as shown by 16S rRNA analysis. However, chlorosomes and BChl c are absent in this bacterial species, and the color of cells resembles that of purple bacteria. The reaction center (RC) complex was isolated from the 'R. castenholzii' cells. The tetra-heme cytochrome subunit of an apparent molecular mass of 35 kDa was bound to the isolated complex, although such a complex has not been isolated in Chloroflexus aurantiacus since its cytochrome subunit (43 kDa) would be easily lost during the RC isolation. The difference in the size of the subunit between the two species was confirmed by the sequence analysis of the cytochrome gene. Three BChl a and three Bphe a were suggested to be bound to a complex as in C. aurantiacus, based on spectral analysis of the complex and the deduced amino acid sequence from the genes for the L and M subunits. The LM genes are located just upstream of the cytochrome gene as in purple bacteria, and the location is different from that of C. aurantiacus. In the region downstream of the cytochrome gene, bchG was found as in C. aurantiacus. The phylogenetic tree of bchG showed that 'R. castenholzii' is the closest to C. aurantiacus but deeply branched from it. These observations suggest that photosynthetic apparatus and genes of 'R. castenholzii' have some intermediate characteristics between purple bacteria and Chloroflexus.