Overexpression of the response regulator rpaA causes an impaired cell division in the Cyanobacterium Synechocystis sp. PCC 6803

Coordination of carbon partitioning and photosynthesis by a two-component signaling network in Synechococcus elongatus PCC 7942

Photosynthetic organisms need to balance the rate of photosynthesis with the utilization of photosynthetic products by downstream reactions. While such "source/sink" pathways are well-interrogated in plants, analogous regulatory systems are unknown or poorly studied in single-celled algal and cyanobacterial species. Towards the identification of energy/sugar sensors in cyanobacteria, we utilized an engineered strain of Synechococcus elongatus PCC 7942 that allows experimental manipulation of carbon status. We conducted a screening of all two-component systems (TCS) and serine/threonine kinases (STKs) encoded in S. elongatus PCC 7942 by analyzing phenotypes consistent with sucrose-induced relaxation of sink inhibition. We narrowed the candidate sensor proteins by analyzing changes observed after sucrose feeding. We show that a clustered TCS network containing RpaA, CikB, ManS and NblS are involved in the regulation of genes related to photosynthesis, pigment synthesis, and Rubisco concentration in response to sucrose. Altogether, these results highlight a regulatory TCS group that may play under-appreciated functions in carbon partitioning and energy balancing in cyanobacteria.

Robust, coherent, and synchronized circadian clock-controlled oscillations along Anabaena filaments

Circadian clocks display remarkable reliability despite significant stochasticity in biomolecular reactions. We study the dynamics of a circadian clock-controlled gene at the individual cell level in Anabaena sp. PCC 7120, a multicellular filamentous cyanobacterium. We found significant synchronization and spatial coherence along filaments, clock coupling due to cell-cell communication, and gating of the cell cycle. Furthermore, we observed low-amplitude circadian oscillatory transcription of kai genes encoding the post-transcriptional core oscillatory circuit and high-amplitude oscillations of rpaA coding for the master regulator transducing the core clock output. Transcriptional oscillations of rpaA suggest an additional level of regulation. A stochastic one-dimensional toy model of coupled clock cores and their phosphorylation states shows that demographic noise can seed stochastic oscillations outside the region where deterministic limit cycles with circadian periods occur. The model reproduces the observed spatio-temporal coherence along filaments and provides a robust description of coupled circadian clocks in a multicellular organism.