Hns Mutant Research Articles

Effects of multicopy LeuO on the expression of the acid-inducible lysine decarboxylase gene in Escherichia coli

We previously reported that mutations in hns, the structural gene for the histone-like protein H-NS, cause derepressed expression of cadA, which encodes the acid-inducible lysine decarboxylase at noninducing pH (pH 8.0). This study reports the characterization of a plasmid isolated from an Escherichia coli library that suppresses the effect of an hns mutation on cadA expression. A previously sequenced open reading frame, leuO, proves to be the gene that causes the hns-complementing phenotype. The mechanism for this phenotype appears to be overexpression of leuO from a multicopy plasmid, which drastically reduces production of CadC, the essential activator for cadA induction. These results show an in vivo regulatory phenotype for leuO, consistent with its proposed protein sequence.

The Escherichia coli DNA-binding protein H-NS is one of the first proteins to be synthesized after a nutritional upshift

Using two-dimensional electrophoresis, the patterns of polypeptide expression of a wild type and an hns mutant of Escherichia coli were examined in exponential and late stationary phases. The same procedure was used for a kinetic study of proteins synthesized during the first 60 min following inoculation into fresh complete medium. The present study focusses on 35 polypeptides differentially synthesized according to the strain and/or the growth phase. One of the most striking and unexpected observations in this work was a strong and transient synthesis of H-NS in the wild-type strain just after the nutritional upshift.

Plasmids bearing hfq and the hns-like gene stpA complement hns mutants in modulating arginine decarboxylase gene expression in Escherichia coli

Biodegradative arginine decarboxylase is inducible by acid and is derepressed in an hns mutant. Several plasmids from an Escherichia coli library that could complement the hns phenotype were characterized and placed into groups. One group includes plasmids that contain the hns gene and are considered true complements. Another group was found to carry the hfq gene, which encodes the host factor HF-1 for bacteriophage Q beta replication. Plasmids of the third group contain inserts that mapped at 60.2 min on the E. coli chromosome. We identified an open reading frame (stpA) with a deduced amino acid sequence showing more than 60% identity with the sequences of H-NS proteins from several species as being responsible for the hns complementing phenotype of the third group.

The H-NS protein is involved in the biogenesis of flagella in Escherichia coli

The function of the flagellum-chemotaxis regulon requires the expression of many genes and is positively regulated by the cyclic AMP-catabolite activator protein (cAMP-CAP) complex. In this paper, we show that motile behavior was affected in Escherichia coli hns mutants. The loss of motility resulted from a complete lack of flagella. A decrease in the level of transcription of the flhD and fliA genes, which are both required for the synthesis of flagella, was observed in the presence of an hns mutation. Furthermore, the Fla- phenotype was not reversed to the wild type in the presence of a cfs mutation which renders the flagellum synthesis independent of the cAMP-CAP complex. These results suggest that the H-NS protein acts as a positive regulator of genes involved in the biogenesis of flagella by a mechanism independent of the cAMP-CAP pathway.

Transcription of the Salmonella typhimurium spv virulence locus is regulated negatively by the nucleoid-associated protein H-NS.

The possibility that the pleiotropic transcriptional regulator H-NS might play a role in regulating expression of the spv virulence locus of Salmonella typhimurium was investigated. A transposon insertion mutation in hns, the gene encoding H-NS, resulted in enhanced transcription of the spvR regulatory gene and the spvB structural gene in stationary phase cultures. Enhanced transcription was not detected prior to stationary phase, indicating that H-NS makes a negative contribution that is growth phase-specific to the control of spv transcription. When H-NS was over-expressed from a multicopy plasmid, the normal stationary phase induction of spv transcription seen in wild-type cells was abolished. spv transcription was also found to be modulated by growth medium osmolarity, a feature common to many H-NS-regulated genes. In addition, transcription of the spv genes was reduced in mutants with abnormal levels of DNA supercoiling.

Role of hns in the virulence phenotype of pathogenic salmonellae

A TnphoA-generated mutant C5060, attenuated for virulence, was derived from the mouse-virulent Salmonella typhimurium strain C5. This mutation, designated hns-112::TnphoA, harbours the transposon in the 3' end of hns, with the alkaline phosphatase open reading frame in the opposite orientation to that of hns. Bacterial strains harbouring hns-112::TnphoA were mucoid and had altered levels of DNA supercoiling, as monitored using pUC18 as a reporter plasmid. Transduction of hns-112::TnphoA into mouse virulent strains, including S. typhimurium SL1344 and Salmonella enteritidis Se795, resulted in attenuation. When an independent hns mutation, harbouring a kanamycin-resistance cassette inserted into the Kpnl site at base pair 237 of the hns gene, was introduced into S. typhimurium C5, the isolates were also attenuated. S. typhimurium C5 isolates harbouring the multicopy plasmid pGB651, which encodes the Escherichia coli hns gene, were partially attenuated in mice. Transductional analysis, using Tn10 insertions located close to the hns gene, showed that virulence could be restored in genetic crosses that eliminated the resident hns mutations. However, some hns+ transductants were still attenuated, suggesting that secondary attenuating lesions can accumulate in hns-deficient strains. These studies show that the hns locus plays a role in Salmonella virulence.

Characterization of broadly pleiotropic phenotypes caused by an hfq insertion mutation in Escherichia coli K-12.

The region immediately downstream from the miaA tRNA modification gene at 94.8 min contains the hfq gene and the hflA region, which are important in the bacteriophage Q beta and lambda life cycles. The roles of these genes in bacteria remain largely unknown. We report here the characterization of two chromosomal hfq insertion mutations. An omega (omega) cassette insertion near the end of hfq resulted in phenotypes only slightly different from the parent, although transcript mapping demonstrated that the insertion was completely polar on hflX expression. In contrast, an equally polar omega cassette insertion near the beginning of hfq caused pronounced pleiotropic phenotypes, including decreased growth rates and yields, decreased negative supercoiling of plasmids in stationary phase, increased cell size, osmosensitivity, increased oxidation of carbon sources, increased sensitivity to ultraviolet light, and suppression of bgl activation by hns mutations. hfq::omega mutant phenotypes were distinct from those caused by omega insertions early in the miaA tRNA modification gene. On the other hand, both hfq insertions interfered with lambda phage plaque formation, probably by means of polarity at the hflA region. Together, these results show that hfq function plays a fundamental role in Escherichia coli physiology and that hfq and the hflA-region are in the amiB-mutL-miaA-hfq-hflX superoperon.

Interactions of the nucleoid-associated DNA-binding protein H-NS with the regulatory region of the osmotically controlled proU operon of Escherichia coli.

The Escherichia coli hns gene encodes the abundant nucleoid-associated DNA-binding protein H-NS. Mutations in hns alter the expression of many genes with unrelated functions and result in a derepression of the proU operon (proVWX) without abolishing the osmotic control of its transcription. We have investigated the interactions of H-NS with the proU regulatory region by deletion analysis of cis-acting sequences, competitive gel retardation assays, and DNase I footprinting. The negative effect of H-NS on proU transcription was mediated by cis-acting sequences within proV but did not depend on the presence of a curved DNA segment upstream of the proU-35 region previously characterized as a target for H-NS binding in vitro. We detected a 46-base pair high affinity H-NS binding region downstream of the proU promoter at the 5' end of the proV gene and a complex array of additional H-NS binding sites which suggest the presence of an extended H-NS nucleoprotein complex. Most of the H-NS binding sites were highly A+T-rich and carried stretches of 5 or more consecutive A-T base pairs. The implications of our results for the osmotic regulation of proU transcription are discussed.

The H-NS protein modulates the activation of the ilvIH operon of Escherichia coli K12 by Lrp, the leucine regulatory protein.

The H-NS protein, the product of the hns gene, plays a central role in the cellular response of bacteria to environmental stresses such as modification of osmolarity and temperature. The leucine regulatory protein (Lrp) controls a wide array of operons both as an activator (e.g. ilvIH) and as a repressor. We demonstrate that H-NS can decrease the activity of Lrp in stationary phase and under conditions of high osmolarity. Strains containing hns mutations have higher levels of Lrp-activated ilvIH transcription, while strains carrying the hns+ allele on a pBR322 plasmid have lower activity of Lrp-directed ilvIH gene expression.

The nucleoid-associated DNA-binding protein H-NS is required for the efficient adaptation of Escherichia coli K-12 to a cold environment.

The hns gene is a member of the cold-shock regulon, indicating that the nucleoid-associated, DNA-binding protein H-NS plays an important role in the adaptation of Escherichia coli to low temperatures. We show here that the ability to cope efficiently with a cold environment (12 degrees C and 25 degrees C) is strongly impaired in E. coli strains carrying hns mutations. Growth inhibition is much more pronounced in strains carrying the hns-206 allele (an ampicillin resistance cassette inserted after codon 37) than in those carrying the hns-205 mutation (a Tn10 insertion located in codon 93). A protein fragment (H-NS*) is synthesized in strains carrying the hns-205::Tn10 mutation, suggesting that this truncated polypeptide is partially functional in the cold adaptation process. Analysis of the growth properties of strains harbouring four different low-copy-number plasmid-encoded hns' genes that result in the production of C-terminally truncated H-NS proteins supports this proposal. H-NS* proteins composed of 133, 117 or 94 amino-terminal amino acids partially complemented the severe cold-sensitive growth phenotype of the hns-206 mutant. In contrast, synthesis of a truncated H-NS protein with only 75 amino-terminal amino acids was insufficient to restore growth at low temperature.

Mutations in a gene encoding a new Hsp70 suppress rapid DNA inversion and bgl activation, but not proU derepression, in hns-1 mutant Escherichia coli

Mutations in hns, the gene encoding the nucleoid-associated protein H-NS, affect both the expression of many specific unlinked genes and the inversion rate of the DNA segment containing the pilA promoter in Escherichia coli. A second-site mutation, termed hscA1, compensated for the effect of an hns-1 mutant allele on the pilA promoter inversion rate and on activation of the bgl operon. The proU operon, induced in an hns-1 background, remained derepressed in an hns-1 hscA1 strain and was induced at an intermediate level in an hns hscA1 strain. An insertion mutant allele, hscA2-cat, conferred the same partial hns-1 compensatory phenotype as the hscA1 allele. The hscA gene encoded a 66-kDa protein product that is a member of the Hsp70 protein class. The gene encoding this product is part of a bicistronic operon that is preceded by a possible sigma 32 promoter and also encodes a 21-kDa protein with significant homology to the DnaJ protein family. The mutation defining the hscA1 allele resulted in a phenylalanine substituting a conserved serine residue located in the ATP-binding region of other Hsp70 proteins.

The RpoS sigma factor relieves H-NS-mediated transcriptional repression of csgA, the subunit gene of fibronectin-binding curli in Escherichia coli.

Curli encoded by the curlin subunit gene, csgA, are fibronectin- and laminin-binding fibres expressed by many natural Escherichia coli and E. coli K-12 strains in response to low temperature, low osmolarity and stationary-phase growth conditions. Curli expression is dependent on RpoS, a sigma factor that controls many stationary phase-inducible genes. Many commonly used K-12 strains carry an amber mutation in rpoS. Strains able to form curli carry an amber suppressor whereas curli-negative E. coli K-12 strains, in general, are sup0. Introduction of SupD, SupE, or supF suppressors into sup0 strains resulted in expression of temperature-regulated curli. In curli-deficient, RpoS- E. coli K-12 strains, csgA is transcriptionally activated by mutations in hns, which encodes the histone-like protein H-NS. Curli expression, fibronectin binding, and csgA transcription remain temperature- and osmoregulated in such double mutants. Our data suggest that RpoS+ strains, and hence curli-proficient strains of E. coli K-12, are relieved for the transcriptional repression mediated by the H-NS protein upon accumulating RpoS as cells reach stationary phase.

Histone-like proteins are required for cell growth and constraint of supercoils in DNA

The gene hns of Escherichia coli K-12, which encodes the histone-like protein H-NS, was inactivated by insertion of a DNA (gene hph) encoding hygromycin phosphotransferase. The growth rates of two mutants lacking either one or the other of the histone-like proteins, HU and IHF, were not affected by introduction of the hns mutation. However, cells depleted of HU, IHF and H-NS simultaneously, could not be constructed by P1 phage-mediated transduction. These results, together with our previous finding that cells deficient in both HU and IHF are viable at 30–37°C [Kano and Imamoto, Gene 89 (1990) 133–137] showed that E. coli cells deficient in any two of these three histone-like proteins are viable at 30–37°C, and suggested that simultaneous deficiency of all three of the proteins is lethal. There were no detectable differences in the levels of superhelicity of the reporter plasmids isolated from cells deficient in either IHF or H-NS, or from wild-type cells, but about 15% decrease in negative superhelicity was detected for the reporter plasmid isolated from cells lacking HU and lacking both HU and H-NS. However, the cryptic bgl operon, whose expression was reported to be regulated through topological changes of cellular DNA, could not be activated in cells depleted of HU or IHF. The bgl operon was expressed in cells depleted of both HU and H-NS as well as in cells depleted of H-NS. These results indicate that expression of the bgl operon is not affected by a change in superhelical density of the cellular DNA and suggest that its expression depends on some physiological processes other than topological alteration of DNA.

Expression and mutational analysis of the nucleoid-associated protein H-NS of Salmonella typhimurium.

The H-NS (H1) protein is a major component of bacterial chromatin. Mutations in the hns (osmZ) gene encoding H-NS are highly pleiotropic, affecting the expression of many unrelated genes in an allele-specific manner. H-NS expression was found not to vary with growth phase or growth medium osmolarity. Additionally, 10 independent hns mutations were isolated and characterized. Five of these mutations were the result of an IS10 insertion, each generating a truncated polypeptide. The other five mutations were the same specific deletion of one amino acid, delta Ala46. The various hns mutations exhibited different phenotypes and influenced DNA topology to variable extents. Implications for the mechanism by which H-NS influences gene expression are discussed.

Lethal overproduction of the Escherichia coli nucleoid protein H-NS: ultramicroscopic and molecular autopsy.

The Escherichia coli hns gene, which encodes the nucleoid protein H-NS, was deprived of its natural promoter and placed under the control of the inducible lambda PL promoter. An hns mutant yielding a protein (H-NS delta 12) with a deletion of four amino acids (Gly112-Arg-Thr-Pro115) was also obtained. Overproduction of wild-type (wt) H-NS, but not of H-NS delta 12, resulted in a drastic loss of cell viability. The molecular events and the morphological alterations eventually leading to cell death were investigated. A strong and nearly immediate inhibition of both RNA and protein synthesis were among the main effects of overproduction of wt H-NS, while synthesis of DNA and cell wall material was inhibited to a lesser extent and at a later time. Upon cryofixation of the cells, part of the overproduced protein was found in inclusion bodies, while the rest was localized by immunoelectron microscopy to the nucleoids. The nucleoids appeared condensed in cells expressing both forms of H-NS, but the morphological alterations were particularly dramatic in those overproducing wt H-NS; their nucleoids appeared very dense, compact and almost perfectly spherical. These results provide direct evidence for involvement of H-NS in control of the organization and compaction of the bacterial nucleoid in vivo and suggest that it may function, either directly or indirectly, as transcriptional repressor and translational inhibitor.