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Abstract VAAM 2012
Molecular basis of symbiosis investigated in Chlorochromatium aggregatum
Petra Henke, Jörg Overmann
Bereich Molekulare Systematik und Diversitätsforschung, Leibniz-Institut DSMZ-Deutsche
Sammlung von Mikroorganismen und Zekllkulturen GmbH, Inhoffenstraße 7B, 38124
Braunschweig
The phototrophic consortium "Chlorochromatium aggregatum" is a mulitcellular association
between the green sulfur bacterial epibionts Chlorobium chlorochromatii and a central motile
chemotrophic Betaproteobacterium. The flagellated central rod moves the entire consortium
towards the light enabling the epibiont to conduct anoxygenic photosynthesis. The cells are
connected through specific cell-cell adhesion structures and division results in two intact
daughter consortia. These observations suggest the exchange of multiple signals between the
epibiont and the central bacterium making this culturable association a suitable system for
understanding the molecular basis of symbiosis between nonrelated bacteria. The comparison
of the Chl. chlorochromatii genome with eleven available genomes of free-living relatives
revealed unique open reading frames. The major fraction of the ORFs code for hypothetical
proteins, but putative large exoproteins and a protein with a RTX toxin-type ß-roll were
identified. In particular Cag1919 which bears several RTX repeats which are typically found
in Gram-negative pathogenic bacteria is of interest. These putative symbiosis genes
(Cag1919, Cag1920, Cag0614 and Cag0616) are constitutively transcribed and have been
analysed further. The whole gene of Cag1919 was cloned into a vector of the pQE series and
expressed heterologously in the E.coli strain XL1Blue. The protein can be used in Ca2+binding experiments due to its predicted Ca2+- binding region. Cag0614 and Cag0616
represent the largest open reading frames in the prokaryotic world known to date with length
of 110418 and 61938 bp, respectively. Due to their large size only fragments can be cloned
and expressed. Interestingly, expression of Cag1919 and 1920 were deleterious to E. coli
strains causing the formation of extremely long, filamented or branched cells. To facilitate the
localization of the proteins in Chl. chlorochromatii, in the free-living and symbiotic state, the
resulting recombinant proteins are used to produce antibodies for immunogold labelling and
tyramide signal amplification. With these results proteins relevant in bacterial symbiosis can
be localized and the question how a motif known from pathogenic bacteria operates in
symbiosis approached.