We analyzed the metabolic rhythms and differential gene expression in the unicellular, diazotrophic cyanobacterium Cyanothece sp. strain ATCC 51142 under N ₂ -fixing conditions after a shift from normal 12-h light-12-h dark cycles to continuous light. We found that the mRNA levels of ∼10% of the genes in the genome demonstrated circadian behavior during growth in free-running (continuous light) conditions. The genes for N ₂ fixation displayed a strong circadian behavior, whereas photosynthesis and respiration genes were not as tightly regulated. One of our main objectives was to determine the strategies used by these cells to perform N ₂ fixation under normal day-night conditions, as well as under the greater stress caused by continuous light. We determined that N ₂ fixation cycled in continuous light but with a lower N ₂ fixation activity. Glycogen degradation, respiration, and photosynthesis were also lower; nonetheless, O ₂ evolution was about 50% of the normal peak. We also demonstrated that nifH (encoding the nitrogenase Fe protein), nifB , and nifX were strongly induced in continuous light; this is consistent with the role of these proteins during the assembly of the enzyme complex and suggested that the decreased N ₂ fixation activity was due to protein-level regulation or inhibition. Many soluble electron carriers (e.g., ferredoxins), as well as redox carriers (e.g., thioredoxin and glutathione), were strongly induced during N ₂ fixation in continuous light. We suggest that these carriers are required to enhance cyclic electron transport and phosphorylation for energy production and to maintain appropriate redox levels in the presence of elevated O ₂ , respectively.

As photosynthetic microbes, cyanobacteria are attractive hosts for the production of high-value molecules from CO₂ and light. Strategies for genetic engineering and tightly controlled gene expression are essential for the biotechnological application of these organisms. Numerous heterologous or native promoter systems were used for constitutive and inducible expression, yet many of them suffer either from leakiness or from a low expression output. Anyway, in recent years, existing systems have been improved and new promoters have been discovered or engineered for cyanobacteria. Moreover, alternative tools and strategies for expression control such as riboswitches, riboregulators or genetic circuits have been developed. In this mini-review, we provide a broad overview on the different tools and approaches for the regulation of gene expression in cyanobacteria and explain their advantages and disadvantages.

Excessive copper concentrations, known to induce reactive oxygen species (ROS) formation, have been tested with respect to their effects on transcript abundance and related proteins involved in oxidative stress responses. The results show that the stromal photosynthetic functions were more ROS sensitive than the membrane‐located reactions. The rbcL over‐expression compensated for the damage only at 10 µm Cu, whereas the genetic stimulation of α‐tocopherol biosynthesis led to the protection of membrane reactions up to 50–100 µm Cu. For this reason, the gradual growth drop of Chlamydomonas reinhardtii cultures observed under increasing Cu²⁺ concentrations matched better with the loss of photosynthetic capacity than with those of photosynthetic quantum yields. According to Larcher’s stress concept, the results allow the identification of gene markers for the alarm (rbcL), the hardening (FeSOD, VTE3) and the exhaustion [cyclin‐dependent protein kinase (cdk), psbA] phases. These genes can be used to rapidly evaluate the state of oxidative stress in algae and putatively in other plant cells.