Molecular Biology

ABSTRACT The complete genome sequence of “ Candidatus Puniceispirillum marinum” IMCC1322, the first cultured representative of the SAR116 clade in the Alphaproteobacteria , is reported here. The genome contains genes for proteorhodopsin, aerobic-type carbon monoxide dehydrogenase, dimethylsulfoniopropionate demethylase, and C 1 compound metabolism. The genome information proposes the SAR116 group to be metabolic generalists in ocean nutrient cycling.

ABSTRACT Protozoa play host for many intracellular bacteria and are important for the adaptation of pathogenic bacteria to eukaryotic cells. We analyzed the genome sequence of “ Candidatus Amoebophilus asiaticus,” an obligate intracellular amoeba symbiont belonging to the Bacteroidetes . The genome has a size of 1.89 Mbp, encodes 1,557 proteins, and shows massive proliferation of IS elements (24% of all genes), although the genome seems to be evolutionarily relatively stable. The genome does not encode pathways for de novo biosynthesis of cofactors, nucleotides, and almost all amino acids. “ Ca . Amoebophilus asiaticus” encodes a variety of proteins with predicted importance for host cell interaction; in particular, an arsenal of proteins with eukaryotic domains, including ankyrin-, TPR/SEL1-, and leucine-rich repeats, which is hitherto unmatched among prokaryotes, is remarkable. Unexpectedly, 26 proteins that can interfere with the host ubiquitin system were identified in the genome. These proteins include F- and U-box domain proteins and two ubiquitin-specific proteases of the CA clan C19 family, representing the first prokaryotic members of this protein family. Consequently, interference with the host ubiquitin system is an important host cell interaction mechanism of “ Ca . Amoebophilus asiaticus”. More generally, we show that the eukaryotic domains identified in “ Ca . Amoebophilus asiaticus” are also significantly enriched in the genomes of other amoeba-associated bacteria (including chlamydiae, Legionella pneumophila , Rickettsia bellii , Francisella tularensis , and Mycobacterium avium ). This indicates that phylogenetically and ecologically diverse bacteria which thrive inside amoebae exploit common mechanisms for interaction with their hosts, and it provides further evidence for the role of amoebae as training grounds for bacterial pathogens of humans.

ABSTRACT ATP/ADP translocases transport ATP across a lipid bilayer, which is normally impermeable to this molecule due to its size and charge. These transport proteins appear to be unique to mitochondria, plant plastids, and obligate intracellular bacteria. All bacterial ATP/ADP translocases characterized thus far have been found in endosymbionts of protozoa or pathogens of higher-order animals, including humans. A putative ATP/ADP translocase was uncovered during the genomic sequencing of the intracellular plant pathogen “ Candidatus Liberibacter asiaticus,” the causal agent of citrus huanglongbing. Bioinformatic analysis of the protein revealed 12 transmembrane helices and predicted an isoelectric point of 9.4, both of which are characteristic of this family of proteins. The “ Ca. Liberibacter asiaticus” gene ( nttA ) encoding the translocase was subsequently expressed in Escherichia coli and shown to enable E. coli to import ATP directly into the cell. Competition assays with the heterologous E. coli system demonstrated that the translocase was highly specific for ATP and ADP but that other nucleotides, if present in high concentrations, could also be taken up and/or block the ability of the translocase to import ATP. In addition, a protein homologous to NttA was identified in “ Ca. Liberibacter solanacearum,” the bacterium associated with potato zebra chip disease. This is the first reported characterization of an ATP translocase from “ Ca. Liberibacter asiaticus,” indicating that some intracellular bacteria of plants also have the potential to import ATP directly from their environment.