Little is known so far about RNA regulators of photosynthesis in plants, algae, or cyanobacteria. The small RNA PsrR1 (formerly SyR1) has been discovered in Synechocystis sp PCC 6803 and appears to be widely conserved within the cyanobacterial phylum. Expression of PsrR1 is induced shortly after a shift from moderate to high-light conditions. Artificial overexpression of PsrR1 led to a bleaching phenotype under moderate light growth conditions. Advanced computational target prediction suggested that several photosynthesis-related mRNAs could be controlled by PsrR1, a finding supported by the results of transcriptome profiling experiments upon pulsed overexpression of this small RNA in Synechocystis sp PCC 6803. We confirmed the interaction between PsrR1 and the ribosome binding regions of the psaL, psaJ, chlN, and cpcA mRNAs by mutational analysis in a heterologous reporter system. Focusing on psaL as a specific target, we show that the psaL mRNA is processed by RNase E only in the presence of PsrR1. Furthermore, we provide evidence for a posttranscriptional regulation of psaL by PsrR1 in the wild type at various environmental conditions and analyzed the consequences of PsrR1-based regulation on photosystem I. In summary, computational and experimental data consistently establish the small RNA PsrR1 as a regulatory factor controlling photosynthetic functions.

Ribonucleases are crucial enzymes in RNA metabolism and post-transcriptional regulatory processes in bacteria. Cyanobacteria encode the two essential ribonucleases RNase E and RNase J. Cyanobacterial RNase E is shorter than homologues in other groups of bacteria and lacks both the chloroplast-specific N-terminal extension as well as the C-terminal domain typical for RNase E of enterobacteria. In order to investigate the function of RNase E in the model cyanobacterium Synechocystis sp. PCC 6803, we engineered a temperature-sensitive RNase E mutant by introducing two site-specific mutations, I65F and the spontaneously occurred V94A. This enabled us to perform RNA-seq after the transient inactivation of RNase E by a temperature shift (TIER-seq) and to map 1472 RNase-E-dependent cleavage sites. We inferred a dominating cleavage signature consisting of an adenine at the −3 and a uridine at the +2 position within a single-stranded segment of the RNA. The data identified mRNAs likely regulated jointly by RNase E and an sRNA and potential 3′ end-derived sRNAs. Our findings substantiate the pivotal role of RNase E in post-transcriptional regulation and suggest the redundant or concerted action of RNase E and RNase J in cyanobacteria.

Twitching motility depends on the adhesion of type IV pili (T4P) to a substrate, with cell movement driven by extension and retraction of the pili. The mechanism of twitching motility, and the events that lead to a reversal of direction, are best understood in rod-shaped bacteria such as Myxococcus xanthus. In M. xanthus, the direction of movement depends on the unipolar localization of the pilus extension and retraction motors PilB and PilT to opposite cell poles. Reversal of direction results from relocalization of PilB and PilT. Some cyanobacteria utilize twitching motility for phototaxis. Here, we examine twitching motility in the cyanobacterium Synechocystis sp. PCC 6803, which has a spherical cell shape without obvious polarity. We use a motile Synechocystis sp. PCC 6803 strain expressing a functional GFP-tagged PilB1 protein to show that PilB1 tends to localize in 'crescents' adjacent to a specific region of the cytoplasmic membrane. Crescents are more prevalent under the low-light conditions that favour phototactic motility, and the direction of motility strongly correlates with the orientation of the crescent. We conclude that the direction of twitching motility in Synechocystis sp. PCC 6803 is controlled by the localization of the T4P apparatus, as it is in M. xanthus. The PilB1 crescents in the spherical cells of Synechocystis can be regarded as being equivalent to the leading pole in the rod-shaped cells.

Blue light controls the expression of genes which are involved in behavioural responses of the cyanobacterium Synechocystis sp. PCC 6803.

The two open reading frames in the Synechocystis sp. PCC 6803 genome, sll1214 and sll1874, here designated cycI and cycII, respectively, encode similar proteins, which are involved in the Mg protoporphyrin monomethylester (MgProtoME) cyclase reaction. The impairment of tetrapyrrole biosynthesis was examined by separate inactivation of both cyclase encoding genes followed by analysis of chlorophyll contents, MgProtoME levels and several enzyme activities of tetrapyrrole biosynthesis. We additionally addressed the question, whether the two isoforms can complement cyclase deficiency under normal aerobic and micro-oxic growth conditions in light. A cycII knock-out mutant grew without any adverse symptoms at normal air conditions, but showed MgProtoME accumulation at growth under low oxygen conditions. A complete deletion of cycI failed in spite of mixotrophic growth and low light at both ambient and low oxygen, but resulted in accumulation of 150 and 28 times more MgProtoME, respectively, and circa 60% of the wild-type chlorophyll content. The CycI deficiency induced a feedback-controlled limitation of the metabolic flow in the tetrapyrrole biosynthetic pathway by reduced ALA synthesis and Fe chelatase activity. Ectopic expression of the CycI protein restored the wild-type phenotype in cycI(-) mutant cells under ambient air as well as micro-oxic growth conditions. Overexpressed CycII protein could not compensate for cycI(-) mutation under micro-oxic and aerobic growth conditions, but complemented the cycII knock-out mutant as indicated by wild-type MgProtoME and chlorophyll levels. Our findings indicate the essential contribution of CycI to the cyclase reaction at ambient and low oxygen conditions, while low oxygen conditions additionally require CycII for the cyclase activity.