Sphäroidkörper in Endosymbiose mit Rhopalodia gibba: Analysen des Genoms und Transkriptstudien der Nitrogenase

The spheroid bodies of the diatom Rhopalodia gibba are obligate cyanobacterial symbionts. 571 042 bp of the spheroid body’s genome were sequenced and analysed. The genome shows many modifications which are correlated with the intracellular lifestyle of the spheroid bodies. These include inter alia the inactivation and the loss of many genes, an increase in the A/T content which mainly affects the non coding regions and the third codon position. Many pseudogenes encoded transposases. Moreover, genes which encoded proteins of the photosynthesis are mostly inactivated. Therefore, the spheroid bodies are probable not able to do photosynthesis. In contrast to the genomes of old endosymbionts, the genome of the spheroid bodies encodes several DNA repair proteins. This is one of several hints that the spheroid bodies are young endosymbionts. The results of the quantitative real time PCR shows that the activity of the nitrogenase in the spheroid bodies is not transcriptional regulated. Furthermore, the nitrogenase fixes molecular nitrogen not only under nitrogen-depleted conditions. This distinguishes the spheroid bodies from their free living relative Cyanothece sp. ATCC 51142. In Cyanothece sp. ATCC 51142 the nitrogen fixation is transcriptional regulated and molecular nitrogen is only fixed under nitrogen-depleted conditions. The investigation of the young Rhopalodia gibba – spheroid body association gives an insight into the degenerative processes which underlie an endosymbiosis in particular at the beginning.