Amino acid sequences of cytochromes c-551 from the halophilic purple phototrophic bacteria, Ectothiorhodospira halophila and E. halochloris

Martin David Kamen (1913-2002): discoverer of carbon 14, and of new cytochromes in photosynthetic bacteria

Martin Kamen was a giant of twentieth century science. Trained as a physical chemist, he was the co-discoverer of radioactive Carbon 14, which has transformed many areas of science as a tracer and as a way to date artifacts. He later switched to the study of metabolism and biochemistry and made important contributions to the understanding of nitrogen fixation and photosynthesis. Finally, he studied cytochromes, primarily from anoxygenic photosynthetic bacteria.

Cytochrome c terminal oxidase pathways of Azotobacter vinelandii: analysis of cytochrome c4 and c5 mutants and up-regulation of cytochrome c-dependent pathways with N2 fixation

The Azotobacter vinelandii cytochrome c5 gene (termed cycB) was cloned and sequenced. Mutants in this c-type cytochrome as well as cytochrome c4 mutants (mutations in cycA) and double mutants in both of the c-type respiratory pathways were characterized. Spectral and heme staining experiments on membranes from the mutants were consistent with the anticipated characteristics of all the gene-directed mutants. Membranes of the individual cytochrome c4 or c5 mutants had normal respiratory rates with physiological substrates but respiration significantly lower than the wild-type rate with ascorbate-N,N,N',N',-tetramethyl-p-phenylenediamine (TMPD) as a reductant. The growth rates of the individual cytochrome c4 or c5 mutants were not markedly different from that of the wild-type strain, but the cycA cycB double-mutant strain was noticeably growth retarded at and below 7.5% O2 on both N-containing and N-free media. The double-mutant strain was unable to grow on agar plates at O2 tensions of 2.5% or less on N-free medium. As the wild-type growth was unaffected by varying the O2 tension, the results indicate that the role of the cytochrome c-dependent pathways is to provide respiration at intermediate (5 to 10%) and low (below 5%) O2 tensions. The two c-type cytochrome genes are transcriptionally up-regulated with N2 fixation; N starvation caused 2.8-fold and 7- to 10-fold increases in the promoter activities of cycA and cycB, respectively, but these activities were affected little by the O2 level supplied to the cultures.

1H and 13C NMR assignments and structural aspects of a ferrocytochrome c-551 from the purple phototrophic bacterium Ectothiorhodospira halophila

Two-dimensional nuclear magnetic resonance was used to assign the 1H and 13C resonances of ferrocytochrome c-551 from Ectothiorhodospira halophila, a halophilic phototrophic purple bacterium. This 78-residue protein belongs to a small subgroup of class I cytochromes c together with the analogous cytochromes c-551 from E. halochloris and E. abdelmalekii. A nearly complete assignment of 13C resonances was obtained at natural abundance using a gradient-enhanced 1H-detected heteronuclear single quantum coherence experiment (HSQC). This was found to be extremely useful for the unambigous assignment of side chain protons. The secondary structure of the protein was determined from analyses of short- and medium-range nuclear Overhauser enhancements (NOE), amide proton exchange and 13C alpha chemical shifts. Three helices could be identified which are well conserved among the class I cytochromes c. There is some evidence for two other regions of less well defined helical structure. From a preliminary analysis of long-range NOE it is shown that in the E. halophila cytochrome c-551 the general cytochrome c fold is well conserved, including the three conserved helices (residues 2-8, 41-50, 63-76), the regions around the heme ligands (Cys14-Ser15-Ser16-Cys17-His18, Met55) and the omega loop (residues 18-28). In addition, three variable segments of the protein are discussed in detail, one of those including a cis-proline, a feature so far unique in the cytochrome c family. Structural alignments of the E. halophila cytochrome c-551 with two other Pseudomonas cytochrome c5 homologs (Azotobacter vinelandii cytochrome c5 and Chlorobium limicola cytochrome c-555) are provided which are based on sequence similarities and secondary structure alignments.

Evolution of photosynthetic reaction centers and light harvesting chlorophyll proteins

It is proposed that there is a single evolutionary origin for photosynthetic reaction centers and also for most light-harvesting chlorophyll proteins. It is generally accepted that the purple bacterial reaction center (quinone-reducing photosystem) and the plant and cyanobacterial PSII (oxygen-evolving photosystem) are homologous. It is also apparent that the green sulfur bacterial reaction center is homologous to cyanobacterial PSI (the pyridine nucleotide reducing photosystem). However, it is less obvious that PSI is related to the purple bacterial reaction center. It is herein proposed that PSI represents a gene fusion of the precursors of small light harvesting bacteriochlorophyll proteins from purple bacteria and purple bacterial reaction centers. Furthermore, it is proposed that reaction centers evolved from the membrane-spanning cytochrome b of the cytochrome bc1 complex and that most membrane-spanning cytochromes may have a common origin.