Functional Assessment Of EnvZ/OmpR Two-component System In Shewanella Oneidensis

Shewanella are facultative gram-negative anaerobes inhabiting a wide variety of niches in nature where they degrade diverse organic compounds and utilize a broad spectrum of electron acceptors for growth under anaerobic conditions. With these features, they are considered important for natural processes like carbon cycling as well as for practical applications such as environmental remediation and microbial fuel cell development. While much of the attention has been given to members of the electron-transport pathways in Shewanella, S. oneidensis in particular, how these bacteria manage to thrive under such a large range of different conditions constantly attracts people’s interest.It has been well known that two-component systems composed of a sensory kinase and a response regulator play central roles in bacterial sensing and adaptation to external environments. Analysis of the genome sequence of S. oneidensis MR-1 revealed that this model organism of Shewanella possesses a two-component-system arsenal one third times bigger than that of Escherichia coli, corroborating the necessity for experimental investigation of these proteins. In this work, we chose to investigate the EnvZ/OmpR two component system which is responsible for osmo-response in E. coli.For the EnvZ/OmpR two component system, we characterized their operon structure and confirmed that their coding genes were co-transcribed as a single polycistronic mRNA using Northern blotting assay. Roles of the EnvZ/OmpR two-component system in response to various stresses were investigated with mutational analysis, quantitative reverse transcriptase PCR (qRT-PCR). Results from the mutational analysis and qRT-PCR suggested that the EnvZ/OmpR system contributed to osmotic stress response of S. oneidensis and very likely engaged a similar strategy employed by E. coli, which involved reciprocal regulation of two porin coding genes. We further showed that the EnvZ/OmpR dependent regulation of porin genes and motility resided almost completely on OmpR and only partially on EnvZ, indicating additional mechanisms for OmpR phosphorylation. In contrast to E. coli lacking ompR-envZ, however, growth of S. oneidensis did not show a significant dependence on this system even under osmotic stress. Taken together, our results suggest that the function of the ompR-envZ system in S. oneidensis, although still connected with osmo-regulation, has diverged considerably from that of E. coli. Additional mechanism must exist to support growth of S. oneidensis under osmotic stress.To summarize, our study investigated function and physiological role of EnvZ/OmpR two-component in S. oneidensis. Despite significant sequence similarities and resonating functions, these two-component systems both differed considerably from their well characterized counterparts in E. coli. For example, the EnvZ/OmpR seems less essential for osmo-response in S1. oneidensis than in E.coli, and probably adopted new role in adaptation to alkaline environment.These results support the postulation that although bacteria rely on similar sets of two-component systems to face and adapt to environmental changes, the specific signals these two-component systems sense and the subsequent response they trigger are probably tailored to their natural habitats to ensure survival. Further and broader analysis of the two-component systems in S. oneidensis should shed light on the basis of the ecological success of this organism and lead to better strategies for engineering and practical application.