Proteomic profiles and gene expressions in the symbiotically competent cyanobacterium Nostoc sp. PCC 73102

Abstract: Nostoc PCC 73102 is an evolutionary important cyanobacterium with multiple phenotypic traits and symbiotic capacities. Based on two-dimensional gel electrophoresis (2-DE) and MALDI-TOF mass spectrometry, a proteomic approach was developed in order to obtain protein profiles of this symbiotically competent cyanobacterium during three trophic types/life stages: free-living photo-autotrophic and diazotrophic growth conditions, dark heterotrophic conditions and hormogonium formation. The proteomic data obtained revealed a total of 82 proteins that could be organized into 12 functional categories. The majority of proteins identified were involved in carbon, nitrogen and energy metabolism. The analysis also indicates that a thioredoxin-dependent redox regulation is vital under photo-autotrophic and diazotrophic growth conditions. The differentiation of vegetative filaments into hormogonia led to distinct morphological as well as drastic changes in C and N metabolism, with levels of Rubisco large subunit and glutamine synthetase clearly down-regulated, while glycogen synthesis (C storage) was enhanced, a metabolic specialisation maintained in symbiosis. RT-PCR and quantitative real-time PCR showed that gene transcription related to surface structures and hormogonium function was affected early during the developmental process: genes encoding proteins involved in motility (pil genes) and gas vesicle formation (gvp genes) were up-regulated, but also genes encoding proteins involved in DNA replication (dnaA) and cell division (ftsZ, ftsA and ftn2) and proteolysis (hetR). Dark heterotrophy, mimicking symbiosis, led to a repression of CO2 fixation, while sugar uptake, the glycolysis pathway and N2-fixation were stimulated. In addition, three proteins involved in light adaptation processes were upregulated. Collectively, these data contribute to our understanding and the definition of ‘symbiotic competence’ and suggest that the cyanobacterium behaves like a cyanobiont even before getting into physical contact with host plants if subject to the appropriate signals.