Chemolithoautotrophic bacteria from the genera Hydrogenovibrio, Thiomicrorhabdus and Thiomicrospira are common, sometimes dominant, isolates from sulfidic habitats including hydrothermal vents, soda and salt lakes and marine sediments. Their genome sequences confirm their membership in a deeply branching clade of the Gammaproteobacteria. Several adaptations to heterogeneous habitats are apparent. Their genomes include large numbers of genes for sensing and responding to their environment (EAL- and GGDEF-domain proteins and methyl-accepting chemotaxis proteins) despite their small sizes (2.1-3.1 Mbp). An array of sulfur-oxidizing complexes are encoded, likely to facilitate these organisms' use of multiple forms of reduced sulfur as electron donors. Hydrogenase genes are present in some taxa, including group 1d and 2b hydrogenases in Hydrogenovibrio marinus and H. thermophilus MA2-6, acquired via horizontal gene transfer. In addition to high-affinity cbb cytochrome c oxidase, some also encode cytochrome bd-type quinol oxidase or ba -type cytochrome c oxidase, which could facilitate growth under different oxygen tensions, or maintain redox balance. Carboxysome operons are present in most, with genes downstream encoding transporters from four evolutionarily distinct families, which may act with the carboxysomes to form CO concentrating mechanisms. These adaptations to habitat variability likely contribute to the cosmopolitan distribution of these organisms.

Staphylococcus aureus is a major human pathogen that causes infection in a wide variety of sites within the human body. Its ability to adapt to the human host and to produce a successful infection requires precise orchestration of gene expression. While DNA-dependent RNA polymerase (RNAP) is generally well characterized, the roles of several small accessory subunits within the complex have yet to be fully explored. This is particularly true for the omega ( or RpoZ) subunit, which has been extensively studied in Gram-negative bacteria but largely neglected in Grampositive counterparts. In Escherichia coli, it has been shown that ppGpp binding, and thus control of the stringent response, is facilitated by . Interestingly, key residues that facilitate ppGpp binding by are not conserved in S. aureus, and consequently, survival under starvation conditions is unaffected by rpoZ deletion. Further to this, -lacking strains of S. aureus display structural changes in the RNAP complex, which result from increased degradation and misfolding of the ␤= subunit, alterations in ␦ and factor abundance, and a general dissociation of RNAP in the absence of . Through RNA sequencing analysis we detected a variety of transcriptional changes in the rpoZ-deficient strain, presumably as a response to the negative effects of depletion on the transcription machinery. These transcriptional changes translated to an impaired ability of the rpoZ mutant to resist stress and to fully form a biofilm. Collectively, our data underline, for the first time, the importance of for RNAP stability, function, and cellular physiology in S. aureus. IMPORTANCEIn order for bacteria to adjust to changing environments, such as within the host, the transcriptional process must be tightly controlled. Transcription is carried out by DNA-dependent RNA polymerase (RNAP). In addition to its major subunits (␣ 2 ␤␤=) a fifth, smaller subunit, , is present in all forms of life. Although this small subunit is well studied in eukaryotes and Gram-negative bacteria, only limited information is available for Gram-positive and pathogenic species. In this study, we investigated the structural and functional importance of , revealing key roles in subunit folding/stability, complex assembly, and maintenance of transcriptional integrity. Collectively, our data underline, for the first time, the importance of for RNAP function and cellular harmony in S. aureus.KEYWORDS RNA polymerase subunit omega, RpoZ, Staphylococcus aureus, gene regulation T ranscription in all forms of life is a tightly controlled process, necessitated by the essentiality of correct temporal and spatial expression of genes for survival. All transcriptional activity within a cell is maintained by the DNA-dependent RNA polymerase (RNAP). This multiprotein complex is structurally and functionally similar in distant forms of life, displaying only minor variations in composition, e.g., the presence/ absence of certain subunits (1, 2). In bacteria RNAP consists of four main subunits, i.e.,