The photochemical reaction center (RC) complex of Roseiflexus castenholzii, which belongs to the filamentous anoxygenic phototrophic bacteria (green filamentous bacteria) but lacks chlorosomes, was isolated and characterized. The genes coding for the subunits of the RC and the light-harvesting proteins were also cloned and sequenced. The RC complex was composed of L, M, and cytochrome subunits. The cytochrome subunit showed a molecular mass of approximately 35 kDa, contained hemes c, and functioned as the electron donor to the photo-oxidized special pair of bacteriochlorophylls in the RC. The RC complex appeared to contain three molecules of bacteriochlorophyll and three molecules of bacteriopheophytin, as in the RC preparation from Chloroflexus aurantiacus. Phylogenetic trees based on the deduced amino acid sequences of the RC subunits suggested that R. castenholzii had diverged from C. aurantiacus very early after the divergence of filamentous anoxygenic phototrophic bacteria from purple bacteria. Although R. castenholzii is phylogenetically related to C. aurantiacus, the arrangement of its puf genes, which code for the light-harvesting proteins and the RC subunits, was different from that in C. aurantiacus and similar to that in purple bacteria. The genes are found in the order pufB, -A, -L, -M, and -C, with the pufL and pufM genes forming one continuous open reading frame. Since the photosynthetic apparatus and genes of R. castenholzii have intermediate characteristics between those of purple bacteria and C. aurantiacus, it is likely that they retain many features of the common ancestor of purple bacteria and filamentous anoxygenic phototrophic bacteria.Photosynthetic prokaryotes consist of anoxygenic photosynthetic bacteria and oxygenic cyanobacteria. Phylogenetic studies based on 16S rRNA gene sequences indicate that photosynthetic prokaryotes are widely distributed in the bacterial lineage, and one group of them, the green filamentous bacteria or filamentous anoxygenic phototrophic bacteria (16, 17), represented by Chloroflexus aurantiacus, is placed on the deepest branch among all photosynthetic bacteria (27). Filamentous anoxygenic phototrophs, therefore, have received considerable attention as an important group when considering the early evolution of photosynthetic organisms (4). The spectroscopic and biochemical properties of the photosynthetic apparatus in the group, however, have been studied only in C. aurantiacus, and the common properties of the photosynthetic apparatus in the filamentous anoxygenic phototrophic bacteria remain unclear.The photochemical reaction center (RC) of C. aurantiacus consists of L, M, and cytochrome subunits and is closely related to those of purple bacteria (5,30,34,35,39). The genes coding for the RC subunits and peripheral light-harvesting (LH) polypeptides are located in two puf operons in C. aurantiacus. One operon includes pufLM, coding for the L and M subunits of the RC complex, respectively (35). The other operon contains pufBAC, coding for the  and ␣ subunits o...