Expression-based identification of genetic determinants of the bacterial symbiosis 'Chlorochromatium aggregatum'

Summary The phototrophic consortium 'Chlorochromatium aggregatum' is a highly structured association of green sulfur bacterial epibionts surrounding a central, motile bacterium and is the most specific symbiosis currently known between two phylogenetically dis- tinct bacterial species. Genes and gene products potentially involved in the symbiotic interaction were identified on the genomic, transcriptomic and pro- teomic level. As compared with the 11 available genomesoffree-livingrelatives,only186openreading frames were found to be unique to the epibiont genome. 2-D differential gel electrophoresis (2-D DIGE) of the soluble proteomes recovered 1612 protein spots of which 54 were detected exclusively in consortia but not in pure epibiont cultures. Using mass spectrometry analyses, the 13 most intense of the 54 spots could be attributed to the epibiont.Analy- ses of the membrane proteins of consortia, of consor- tia treated with cross-linkers and of pure cultures indicated that a branched chain amino acid ABC- transporter binding protein is only expressed in the symbiotic state of the epibiont. Furthermore, analyses of chlorosomes revealed that an uncharacterized 11 kDa epibiont protein is only expressed during sym- biosis.Thisproteinmaybeinvolvedintheintracellular sorting of chlorosomes. Application of a novel prokaryotic cDNA suppression subtractive hybridiza- tion technique led to identification of 14 differentially regulated genes, and comparison of the transcrip- tomes of symbiotic and free-living epibionts indicated that 328 genes were differentially transcribed. The three approaches were mostly complementary and thereby yielded a first inventory of 352 genes that are likely to be involved in the bacterial interaction in 'C. aggregatum'. Notably, most of the regulated genes encoded components of central metabolic pathways whereas only very few (7.5%) of the unique 'symbiosis genes'turnedouttoberegulatedundertheexperimen- tal conditions tested. This pronounced regulation of central metabolic pathways may serve to fine-tune the symbioticinteractionin'C.aggregatum'inresponseto environmental conditions.