## 4 Discussion
The upregulation of _ftsH_ observed in our study under chill/light stress after 24 h (which was prolonged in the following days) suggests that in AMU-DH-30 _ftsH_ supports the assembly of protein complexes in the photosynthetic electron-transport pathways. Other genes that are required to support the photosynthetic activity at a low temperature, crucial in the response to chill/light stress belong to _ccr_ (cyanobacterial cold resistance genes) [15; 16]. Their expression can be influenced by Rbp proteins [38] as discussed above. Ccr2 has previously been reported as a representative of a novel thylakoid protein required for survival in lower temperatures [16]. Our results seem to confirm the importance of this protein in the adaptation to lower temperature. The increased level of _ccr2_ observed in AMU-DH-30 strain may affect a membrane functionality and the PSII-mediated electron transport rate, by interaction with PSII or electron carriers, as was suggested by [PERSON] et al. [16]. Other metabolic processes important in the response to low temperature are those triggering the protein synthesis inhibited under low temperature. The upregulation of such genes required to reinitiate the translation (such as _nusG_ and _hlfX_) occur after prolonged to low temperature [40; 41]. A conserved transcription factor NusG may participate in transcription attenuation and translational  
Figure 5: Schematic indicating the role of analyzed genes in a cyanobacterial cell and their response to chill/light stress. Red and blue arrows represent the gene upregulation in AMU-DH-30 and CS-505 strains, respectively (solid—indicates early response (1 h), dotted—response after prolonged exposure to stress (120 h), bold arrows—at least 4-fold, thus biologically relevant upregulation). Rbp1—RNA-binding protein, involved in posttranscriptional regulation; DesA—delta(12)-fatty-acid desaturase, and FabZ (3R)—hydroxymyristoyl-[acyl-carrier-protein]-dehydratase, both involved in unsaturation of fatty acids of the membrane lipids; FtsH—ATP-dependent zinc metalloprotease responsible for the thylakoid membranes biogenesis and control of the photosystem II repair cycle; Ccr2—assumable thylakoid protein important in maintaining the membrane functionality and affecting the PS II-mediated electron transport rate; NusG—a transcription factor that may participate in transcription attenuation and translational control mechanisms; HfIX—GTPase, a dissociation factor of the 100S ribosome that plays a role in the regulation of 70S and 100S ribosome homeostasis; Asd—aspartate-beta-semialdehyde dehydrogenase, enzyme involved in lysine biosynthetic pathway and the further accumulation of AASA participating in stress response; CyrB—enzyme involved in cytotoxic CYN synthesis pathway. See details in the main text. control mechanisms [43]. Its expression in AMU-DH-30 increased after 1 h of chill/light stress and was upregulated after 120 h, as well, suggesting that the transcriptional and translational machineries can be controlled by the mechanism involving NusG. The expression of _hflX_ in this strain was significantly increased in the same manner. Conserved GTPase HfIX has recently been found as a dissociation factor of the _S. aureus_ 100S ribosome [44] that plays a key role in the regulation of 70S and 100S ribosome homeostasis, which is closely connected with the bacterial survival during different stress conditions. The expression of _hflX_, that may be regulated in a temperature-dependent manner [45], is necessary to reuse the ribosome for new cycles of translation. The upregulation of _hflX_ observed in our study may promote the 70S ribosomes disassembly and suggests that _R. raciborskii_ AMU-DH-30 cells can maintain cell viability by reversible conversion of silent 100S to a translationally active 70S ribosome. An alternative and widely used protective strategy to overcome the challenges of a changing environment is based on the accumulation of low-molecular-weight, water-soluble, organic compounds, known as compatible solutes [46].