Regulation of a polyamine transporter by the conserved 3′ UTR-derived sRNA SorX confers resistance to singlet oxygen and organic hydroperoxides in <i>Rhodobacter sphaeroides</i>

Singlet oxygen is generated by bacteriochlorophylls when light and oxygen are simultaneously present in <i>Rhodobacter sphaeroides</i>. Singlet oxygen triggers a specific response that is partly regulated by the alternative sigma factor RpoHI/HII. The sRNA RSs2461 has previously been identified as an RpoHI/HII-dependent sRNA and is derived from the 3′ UTR of the mRNA for an OmpR-type transcriptional regulator. Similar to the RpoHI/HII-dependent CcsR and SorY sRNAs, RSs2461 affects the resistance of <i>R. sphaeroides</i> against singlet oxygen and was therefore renamed here SorX. Furthermore, SorX has a strong impact on resistance against organic hydroperoxides that usually occur as secondary damages downstream of singlet oxygen. The 75-nt SorX 3′ fragment, which is generated by RNase E cleavage and highly conserved among related species, represents the functional entity. A target search identified <i>potA</i> mRNA, which encodes a subunit of a polyamine transporter, as a direct SorX target and stress resistance via SorX could be linked to <i>potA</i>. The PotABCD transporter is an uptake system for spermidine in <i>E. coli</i>. While spermidine is generally described as beneficial during oxidative stress, we observed significantly increased sensitivity of <i>R. sphaeroides</i> to organic hydroperoxides in the presence of spermidine. We therefore propose that the diminished import of spermidine, due to down-regulation of <i>potA</i> by SorX, counteracts oxidative stress. Together with results from other studies this underlines the importance of regulated transport to bacterial stress defense.