The data obtained in the presented work again emphasise the importance of bacterial sRNA as regulators that fine-tune the metabolism and promote the stress defence upon changes in the environment (reviewed in Wagner and Romby, 2015). For the investigated model R. sphaeroides, a microorganism displaying a great metabolic versatility, many proteinaceous factors in the adaptation and response to environmental stimuli are known which mainly comprise receptors, transcription factors, and alternative sigma factors (described in 1.4, 1.6, 1.9.2, and 1.9.4). Since the discovery of a vast set of sRNAs in 2009 (Berghoff et al., 2009), the function of four of these sRNAs, namely PcrZ, CcsR1-4, SorY, and SorX, has been described (1.9.5), giving an even more detailed overview of the adaptation abilities of R. sphaeroides. In line with these descriptions, the here investigated sRNAs Pos19, PcrX, and RSs0827 affect adaptive metabolic changes as well as the cells response in the defence against oxidative stress. The sRNA Pos19 was found to affect pathways in the sulphur- and following glutathione (GSH) synthesis, by repressing a set of sulphur genes and the hypothetical protein RSP_0557. For the latter, a balanced expression seems to be required for a balanced cellular GSH pool. By means of these fine-tuning aspects of the sulphur metabolism that result in balanced cysteine and GSH levels, the RpoE-dependent sRNA Pos19 contributes to the refined response to oxidative stress. Additionally, Pos19 encodes a small peptide whose function so far remains enigmatic. Still, the survey concerning the peptide translation indicated that in R. sphaeroides the Shine-Dalgarno sequence is not essential for translation initiation, as it is known for other prokaryotes (Omotajo et al., 2015). The sRNA PcrX was found to be encoded in and processed from the 3 UTR of the puf mRNA. In the last years more and more focus was put on 3 UTR derived sRNAs (reviewed in Miyakoshi et al., 2015). Interestingly, PcrX was found to regulate its parental mRNA via binding to the pufX part of the polycistronic mRNA. The obtained data hint towards the hypothesis that PcrX is involved in the segmental stability of the puf mRNA which is needed to ensure the translation of the encoded photosynthetic components in a stoichiometric amount. Another facet of the sRNA besides the selfcontrol of its parental mRNA could be the regulation of additional genes. This idea is supported by the findings that the PcrX over-expression affects the mRNA levels of bchN and bchY, and that PcrX was found to be Hfq-bound, even though the effect on pufX seems to be independent of Hfq. Taken together, PcrX displays the second sRNA involved in the photosynthesis gene regulation of R. sphaeroides besides PcrZ. Regarding the sRNA RSs0827, the newly obtained data strongly support the finding that the sRNA interacts with and thereby regulates the rpoE mRNA. Therefore RSs0827 could display a potential regulator that ensures the attenuation of the RpoE-dependent stress response under certain conditions. So far an effect of RSs0827 on the RpoE-dependent sRNA Pos19 could only be found during the stationary phase, but not during conditions of photo-oxidative stress.
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