title: Novel insights into the origin and diversification of photosynthesis based on analyses of conserved indels in the core reaction center proteins.
abstract: The evolution and diversification of different types of photosynthetic reaction centers (RCs) remains an important unresolved problem. We report here novel sequence features of the core proteins from Type I RCs (RC-I) and Type II RCs (RC-II) whose analyses provide important insights into the evolution of the RCs. The sequence alignments of the RC-I core proteins contain two conserved inserts or deletions (indels), a 3 amino acid (aa) indel that is uniquely found in all RC-I homologs from Cyanobacteria (both PsaA and PsaB) and a 1 aa indel that is specifically shared by the Chlorobi and Acidobacteria homologs. Ancestral sequence reconstruction provides evidence that the RC-I core protein from Heliobacteriaceae (PshA), lacking these indels, is most closely related to the ancestral RC-I protein. Thus, the identified 3 aa and 1 aa indels in the RC-I protein sequences must have been deletions, which occurred, respectively, in an ancestor of the modern Cyanobacteria containing a homodimeric form of RC-I and in a common ancestor of the RC-I core protein from Chlorobi and Acidobacteria. We also report a conserved 1 aa indel in the RC-II protein sequences that is commonly shared by all homologs from Cyanobacteria but not found in the homologs from Chloroflexi, Proteobacteria and Gemmatimonadetes. Ancestral sequence reconstruction provides evidence that the RC-II subunits lacking this indel are more similar to the ancestral RC-II protein. The results of flexible structural alignments of the indel-containing region of the RC-II protein with the homologous region in the RC-I core protein, which shares structural similarity with the RC-II homologs, support the view that the 1 aa indel present in the RC-II homologs from Cyanobacteria is a deletion, which was not present in the ancestral form of the RC-II protein. Our analyses of the conserved indels found in the RC-I and RC-II proteins, thus, support the view that the earliest photosynthetic lineages with living descendants likely contained only a single RC (RC-I or RC-II), and the presence of both RC-I and RC-II in a linked state, as found in the modern Cyanobacteria, is a derivation from these earlier phototrophs.
authors.affiliation: Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, ON, L8N 3Z5, Canada.
authors.forename: Bijendra
authors.initials: B
authors.lastname: Khadka
authors.affiliation: Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, ON, L8N 3Z5, Canada.
authors.forename: Mobolaji
authors.initials: M
authors.lastname: Adeolu
authors.affiliation: Department of Biology and Department of Chemistry, Washington University in St. Louis, St. Louis, MO, 63130, USA.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Blankenship
authors.affiliation: Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, ON, L8N 3Z5, Canada. gupta@mcmaster.ca.
authors.forename: Radhey S
authors.initials: RS
authors.lastname: Gupta
mesh_terms: D000595:Amino Acid Sequence; D054643:INDEL Mutation; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000235:genetics*; D010802:Phylogeny; D017386:Sequence Homology, Amino Acid
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins
keywords: Ancestral sequence reconstruction; Conserved indels; Evolution of photosynthesis; Reaction center core proteins; Structural alignments