Role for the histone-like protein H-NS in growth phase-dependent and osmotic regulation of sigma S and many sigma S-dependent genes in Escherichia coli

Abstract

TheS subunit of RNA polymerase (encoded by the rpoS gene) is the master regulator in a complex regulatory network that controls stationary-phase induction and osmotic regulation of many genes in Escherichia coli. Here we demonstrate that the histone-like protein H-NS is also a component of this network, in which it functions as a global inhibitor of gene expression during the exponential phase of growth. On two-dimensional gels, at least 22 S -controlled proteins show increased expression in an hns mutant. H-NS also inhibits the expression of S itself by a mechanism that acts at the posttranscriptional level. Our results indicate that relief of repression by H-NS plays a role in stationary-phase induction as well as in hyperosmotic induction of rpoS translation. Whereas certain S -dependent genes (e.g., osmY) are only indirectly regulated by H-NS via its role in the control of S expression, others are also H-NS-regulated in a S -independent manner. (For this latter class of genes, rpoS hns double mutants show higher levels of expression than mutants deficient in rpoS alone.) In addition, we demonstrate that the slow-growth phenotype of hns mutants is suppressed in hns rpoS double mutants and that many second-site suppressor mutants that spontaneously arise from hns strains carry lesions that affect the expression of S . TheS subunit of RNA polymerase acts as a master regulator in a regulatory network that controls the expression of numerous stationary-phase-induced and osmotically regulated genes in Escherichia coli (11,12,15). rpoS, the structural gene for S , is itself induced during entry into the stationary phase (22,23,25,30,34,39) and in response to an increase in medium osmolarity (23). Under both conditions, control of the cellular S level is largely posttranscriptional, involving stimulation of translation (23, 30) as well as changes in S stability (23,43). So far, more than 30 genes or operons that are under direct or indirect control of S have been identified. Within this large S regulon, differential regulation has been observed for subsets of genes, for instance in response to anaerobiosis (3, 5, 6) or oxidative stress (1, 26). In addition, during entry into the stationary phase, the induction of various S -dependent genes follows different kinetics. These observations indicate that additional factors besides S participate in the fine regulation of these genes. The cyclic AMP (cAMP)-cAMP receptor protein complex (13,20,21,29,47), integration host factor (1, 20), and Lrp (20) have been identified as modulating factors in the control of various S -dependent genes. The data presently available indicate that various combinations of regulatory factors that can act either positively or negatively are used for the control of various stationary-phaseinducible genes. This regulatory strategy results in a high degree of specific fine modulation of S -controlled genes with respect to the time of induction during entry into the stationary phase and in response to additional environmental parameters.In the prese...

The LysR‐like regulator LeuO in Escherichia coli is involved in the translational regulation of rpoS by affecting the expression of the small regulatory DsrA‐RNA

“…evolved second-site suppressor mutants. These strains yielded highly variable ␤-galactosidase activities obviously dependent on the specific nature of the second-site suppressor strains that took over the cultures when cells were grown in liquid medium [this effect was much more pronounced than in mutants deficient in hns alone, in which at least fresh transductants can be reliably assayed (Barth et al, 1995)]. …”

“…This factor is the LysR-like regulator, LeuO, which is shown to reduce rpoS translation mainly at low temperature by inhibiting the expression of DsrA-RNA. In addition, we found that H-NS is a potent repressor or even silencer of leuO, indicating that H-NS, while having a strong and general inhibitory role in the control of rpoS translation (Barth et al, 1995;Yamashino et al, 1995), also positively affects the level of DsrA-RNA, i.e. an activator of rpoS translation, by maintaining low levels of the dsrA repressor LeuO.…”

“…The RNA-binding protein, Hfq (HF-I), previously known as a host factor for the replication of phage Q␤ RNA, has been found to be crucial for rpoS translation, both in E. coli (Muffler et al, 1996b) and in Salmonella (Brown and Elliott, 1996). H-NS protein, even though known as a histone-like nucleoid protein, is involved in the post-transcriptional control of S , in which it somehow interferes with rpoS translation and stimulates S proteolysis (Barth et al, 1995;Yamashino et al, 1995). The small regulatory DsrA-RNA was reported to antagonize repression by H-NS and to be essential for low-temperature induction of rpoS translation (Sledjeski and Gottesman, 1995;Sledjeski et al, 1996).…”

H‐NS: a modulator of environmentally regulated gene expression

“…The amount of S increases in high-osmolarity medium, upon entry into stationary phase, at the onset of carbon or phosphate starvation and at low temperature (Lange and Hengge-Aronis, 1994;Gentry et al, 1993, Sledjeski et al, 1996. In hns mutants the concentration of S is increased more than 10-fold relative to a wild-type strain grown exponentially at low osmolarity (Yamashino et al, 1995;Barth et al, 1995). The increased expression in hns mutants of a number of genes transcribed in a S -dependent manner might therefore be affected indirectly by the increased S concentration (Table 1).…”

“…In general they grow more slowly than wild-type strains (Barth et al, 1995;Yamashino et al, 1995), they show increased osmosensitivity (Barth et al, 1995) and a prominent growth lag upon a shift to low temperature . A key to how hns mutants can display such pleitropic phenotypes was obtained when it was demonstrated that H-NS affects transcription of many unrelated genes and that the level of synthesis of more than 30 proteins is changed in hns mutants (Barth et al, 1995;Bertin et al, 1990;Yamada et al, 1991). Furthermore, H-NS is involved, directly or indirectly, in many other biological processes (see Ussery et al (1994) for a review).…”