# Involvement of _E. coli_ 6S RNA in Oxidative Stress Response
## 1 Introduction
coli_ 6S RNA) in the strain 16S caused a delay in the outgrowth of cell cultures from the stationary phase and resulted in earlier sporulation [25; 26]. Proteome analyses of _B. subtilis_ PY79 derivative strains with knockouts of genes encoding its two 6S RNA paralogs, 6S-1 and 6S-2 RNA, also revealed changes in expression levels of many proteins involved in metabolisms of purines, amino acids and carbohydrates, including a number of stress response regulators, such as _guB_, _cysK_, _tpx_ (superoxide-inducible proteins), _yvyD_ and _yjID_ (glucose starvation-inducible proteins), _rpl_] (cold shock and salt stress protein), _greA_, _yraA_, _ahpC_, _sodA_ and _nadE_ (general stress proteins) [19]. Recently _B. subtilis_ 6S-2 RNA, but not 6S-1 RNA, was demonstrated to regulate biofilm formation, swarming activity and sporulation in the undomestic wild-type strain NCIB 3610 [27], phenotypes that were not observable in 6S RNA knockout strains derived from the laboratory strain PY79 [19]. Noteworthy, and in contrast to the 6S RNA double knockout in the _B. subtilis_ PY79 background (see above), deletion of both 6S RNA genes in strain NCIB 3610 retarded outgrowth under alkaline stress (pH 9.5) relative to the parental and single knockout derivative strains [27]. Evidence for a key role of 6S RNA in the regulation of bacterial stress responses has also been growing owing to studies in other bacteria beyond _E. coli_ and _B. subtilis_. For example, lack of 6S RNA was reported to lead to slower growth of _Rhodobacter sphaeroides_ under high salt stress (250 mM NaCl) [28], to delay recovery of _Synechocystis_ sp. PCC 6803 cells from nitrogen starvation [29] and to reduce survival of _Salmonella enterica_ serovar Typhimurium under acidic stress (pH 3.0) [30]. Moreover, in the latter case, the knockout strain had a reduced ability to invade HeLa cells and showed attenuated virulence in a mouse model [30]. Similar reductions of bacterial pathogenicity upon 6S RNA gene deletion were observed for _L. pneumoniae_ during infection of the protist _Acanthamoeba castellanii_ or mammalian THP-1 cells [31], and for the Lyme disease-causing spirochete _Borrelia burgdorferi_ in a mouse infection model [32]. In summary, 6S RNA functions seem to be quite diverse across different bacterial phyla, possibly species-specific in some cases. Yet, an emerging commonality of 6S RNAs is their importance for bacterial physiology, especially under adverse environmental conditions. Surprisingly, 6S RNA knockouts have so far not been reported to be lethal for their bacterial host, despite the RNA's function as a global regulator of transcription. Seemingly, 6S RNAs are intertwined with additional, partly redundant mechanisms of gene expression control that can compensate for the loss of 6S RNA to some extent. In the present work, we observed the involvement of 6S RNA in the oxidative stress response in _E. coli_ and report the first-ever lethal phenotype of a 6S RNA-deficient strain at high concentrations of hydrogen peroxide. Notably, _E. coli_ 6S RNA knockouts have been studied since 1985 [33], but no data for such conditions were reported. Thus, our work has discovered a novel pathway of 6S RNA-mediated regulation of the _E. coli_ stress response that is critical for cell survival.