Mutations conferring resistance to azide in Escherichia coli occur primarily in the secA gene

NaClO Co-selects antibiotic and disinfectant resistance in Klebsiella pneumonia: Implications for the potential risk of extensive disinfectant use during COVID-19 pandemic

The inappropriate use of antibiotics is widely recognized as the primary driver of bacterial antibiotic resistance. However, less attention has been given to the potential induction of multidrug-resistant bacteria through exposure to disinfectants. In this study, Klebsiella pneumonia, an opportunistic pathogen commonly associated with hospital and community-acquired infection, was experimentally exposed to NaClO at both minimum inhibitory concentration (MIC) and sub-MIC levels over a period of 60 days. The result demonstrated that NaClO exposure led to enhanced resistance of K. pneumonia to both NaClO itself and five antibiotics (erythromycin, polymyxin B, gentamicin, tetracycline, and ciprofloxacin). Concurrently, the evolved resistant strains exhibited fitness costs, as evidenced by decreased growth rates. Whole population sequencing revealed that both concentrations of NaClO exposure caused genetic mutations in the genome of K. pneumonia. Some of these mutations were known to be associated with antibiotic resistance, while others had not previously been identified as such. In addition, 11 identified mutations were located in the virulence factors, demonstrating that NaClO exposure may also impact the pathogenicity of K. pneumoniae. Overall, this study highlights the potential for the widespread use of NaClO-containing disinfectants during the COVID-19 pandemic to contribute to the emergence of antibiotic-resistant bacteria. ENVIRONMENTAL IMPLICATION: Considering the potential hazardous effects of disinfectant residues on environment, organisms and biodiversity, the sharp rise in use of disinfectants during COVID-19 pandemic has been considered highly likely to cause worldwide secondary disasters in ecosystems and human health. This study demonstrated that NaClO exposure enhanced the resistance of K. pneumonia to both NaClO and five antibiotics (erythromycin, polymyxin B, gentamicin, tetracycline, and ciprofloxacin), highlighting the widespread use of NaClO-containing disinfectants during the COVID-19 pandemic may increase the emergence of antibiotic-resistant bacteria in the environment.

Hazard of polystyrene micro-and nanospheres to selected aquatic and terrestrial organisms

Plastics contamination in the environment is a major concern. Risk assessment of micro- and nanoplastics (MPL and NPL) poses significant challenges due to MPL and NPL heterogeneity regarding compositional polymers, particle sizes and morphologies in the environment. Yet, there exists considerable toxicological literature on commercial polystyrene (PS) micro- and nanospheres. Although such particles do not directly represent the environmental MPL and NPL, their toxicity data should be used to advance the hazard assessment of plastics. Here, toxicity data of PS micro- and nanospheres for microorganisms, aquatic and terrestrial invertebrates, fish, and higher plants was collected and analyzed. The evaluation of 294 papers revealed that aquatic invertebrates were the most studied organisms, nanosized PS was studied more often than microsized PS, acute exposures prevailed over chronic exposures, the toxicity of PS suspension additives was rarely addressed, and ∼40 % of data indicated no organismal effects of PS. Toxicity mechanisms were mainly studied in fish and nematode Caenorhabditis elegans, providing guidance for relevant studies in higher organisms. Future studies should focus on environmentally relevant plastics concentrations, wide range of organisms, co-exposures with other pollutants, and method development for plastics identification and quantification to fill the gap of bioaccumulation assessment of plastics.

Iron is a ligand of SecA-like metal-binding domains in vivo

The ATPase SecA is an essential component of the bacterial Sec machinery, which transports proteins across the cytoplasmic membrane. Most SecA proteins contain a long C-terminal tail (CTT). In Escherichia coli, the CTT contains a structurally flexible linker domain and a small metal-binding domain (MBD). The MBD coordinates zinc via a conserved cysteine-containing motif and binds to SecB and ribosomes. In this study, we screened a high-density transposon library for mutants that affect the susceptibility of E. coli to sodium azide, which inhibits SecA-mediated translocation. Results from sequencing this library suggested that mutations removing the CTT make E. coli less susceptible to sodium azide at subinhibitory concentrations. Copurification experiments suggested that the MBD binds to iron and that azide disrupts iron binding. Azide also disrupted binding of SecA to membranes. Two other E. coli proteins that contain SecA-like MBDs, YecA and YchJ, also copurified with iron, and NMR spectroscopy experiments indicated that YecA binds iron via its MBD. Competition experiments and equilibrium binding measurements indicated that the SecA MBD binds preferentially to iron and that a conserved serine is required for this specificity. Finally, structural modeling suggested a plausible model for the octahedral coordination of iron. Taken together, our results suggest that SecA-like MBDs likely bind to iron in vivo.

Growth arrest and a persister state enable resistance to osmotic shock and facilitate dissemination of Vibrio cholerae

Vibrio cholerae is a water-borne bacterial pathogen and causative agent of cholera. Although V. cholerae is a halophile, it can survive in fresh water, and this has a major role in cholera epidemics through consumption of contaminated water and subsequent fecal-oral spread. After dissemination from humans back into fresh water, V. cholerae encounters limited nutrient availability and an abrupt drop in conductivity but little is known about how V. cholerae adapts to, and survives in this environment. In this work, by abolishing or altering the expression of V. cholerae genes in a high-throughput manner, we observed that many osmotic shock tolerant mutants exhibited slowed or arrested growth, and/or generated a higher proportion of persister cells. In addition, we show that growth-arrested V. cholerae, including a persister subpopulation, are generated during infection of the intestinal tract and together allow for the successful dissemination to fresh water. Our results suggest that growth-arrested and persister subpopulations enable survival of V. cholerae upon shedding to the aquatic environment.

Characterization of carbapenem-resistant Acinetobacter baumannii isolates in a Chinese teaching hospital

Carbapenem-resistant Acinetobacter baumannii (CRAB) presents a serious therapeutic and infection control challenge. In this study, we investigated the epidemiological and molecular differences of CRAB and the threatening factors for contributing to increased CRAB infections at a hospital in western China. A total of 110 clinical isolates of A. baumannii, collected in a recent 2-year period, were tested for carbapenem antibiotic susceptibility, followed by a molecular analysis of carbapenemase genes. Genetic relatedness of the isolates was characterized by multilocus sequence typing. Sixty-seven of the 110 isolates (60.9%) were resistant to carbapenems, 80.60% (54/67) of which carried the bla_OXA-23 gene. Most of these CRAB isolates (77.62%) were classified as clone complex 92 (CC92), and sequence type (ST) 92 was the most prevalent STs, followed by ST195, ST136, ST843, and ST75. One CRAB isolate of ST195 harbored plasmid pAB52 from a Chinese patient without travel history. This plasmid contains toxin–antitoxin elements related to adaptation for growth, which might have emerged as a common vehicle indirectly mediating the spread of OXA-23 in CRAB. Thus, CC92 A. baumannii carrying OXA-23 is a major drug-resistant strain spreading in China. Our findings indicate that rational application of antibiotics is indispensable for minimizing widespread of drug resistance.

Subunit a of the F(1)F(0) ATP synthase requires YidC and SecYEG for membrane insertion

The insertion of inner membrane proteins in Escherichia coli occurs almost exclusively via the SecYEG pathway, while some membrane proteins require the membrane protein insertase YidC. In vitro analysis demonstrates that subunit a of the F(1)F(0) ATP synthase (F(0)a) is strictly dependent on Ffh, SecYEG and YidC for its membrane insertion but independent of the proton motive force. The insertion of the first transmembrane segment of F(0)a also depends on Ffh and SecYEG but not on YidC, whereas the insertion is strongly dependent on the proton motive force, unlike the full-length F(0)a protein. These data demonstrate an extensive role of YidC in the assembly of the F(0) sector of the F(1)F(0) ATP synthase.

The T4 RI antiholin has an N-terminal signal anchor release domain that targets it for degradation by DegP

Lysis inhibition (LIN) of T4-infected cells was one of the foundational experimental systems for modern molecular genetics. In LIN, secondary infection of T4-infected cells results in a dramatically protracted infection cycle in which intracellular phage and endolysin accumulation can continue for hours. At the molecular level, this is due to the inhibition of the holin, T, by the antiholin, RI. RI is only 97 residues and contains an N-terminal hydrophobic domain and a C-terminal hydrophilic domain; expression of the latter domain fused to a secretory signal sequence is sufficient to impose LIN, due to its specific interaction with the periplasmic domain of the T holin. Here we show that the N-terminal sequence comprises a signal anchor release (SAR) domain, which causes the secretion of RI in a membrane-tethered form and then its subsequent release into the periplasm, without proteolytic processing. Moreover, the SAR domain confers both functional lability and DegP-mediated proteolytic instability on the released form of RI, although LIN is not affected in a degP host. These results are discussed in terms of a model for the activation of RI in the establishment of the LIN state.

Peptidoglycan lytic activity of thePseudomonas aeruginosaphage φKZ gp144 lytic transglycosylase

The gp144 endolysin gene from the Pseudomonas aeruginosa phage phiKZ was cloned and studies of gp144 expression into Escherichia coli showed host cell lysis. The gp144 protein was purified directly from the culture supernatant and from the bacterial cell pellet and showed in vitro antibacterial lytic activity against P. aeruginosa bacteria and degraded purified peptidoglycan of Gram-negative bacteria. MS analysis identified the gp144 peptidoglycan cleavage site and confirmed a lytic transglycosylase enzyme. Studies of gp144 expression in the presence of sodium azide (NaN(3)), an inhibitor of the protein export machinery, and into an E. coli MM52 secA(ts) mutant at permissive and restrictive temperatures showed that gp144 was secreted independently of the Sec system. The solution conformation of purified gp144 analyzed by circular dichroism spectroscopy was 61% in alpha-helical content, and showed a 72% decrease when interacting with dimyristoylphosphatidylglycerol (DMPG), one of the major components of bacterial membranes and less than 10% with dimyristoylphosphatidylcholine (DMPC) found in eukaryotic membranes. Membrane vesicles of DMPG anionic lipids containing calcein indicated that gp144 caused a rapid release of fluorescent calcein when interacting with synthetic membranes. These results indicated that gp144 from phiKZ is a lytic transglycosylase capable of interacting with and disorganizing bacterial membranes and has potential as an antipseudomonal in phage therapy.

SsrA tagging of Escherichia coli SecM at its translation arrest sequence

SecM is expressed from the secM-secA operon and activates the expression of secA in response to secretion defects. The 3'-end of secM encodes an "arrest sequence," which can interact with the ribosomal exit tunnel, preventing complete secM translation under secretion-defective conditions. In a cis-acting manner, ribosome stalling enhances secA translation. Pro(166) is the last residue incorporated when SecM elongation is arrested. We report that secretion deficiencies lead to SsrA tagging of SecM after Pro(166), Gly(165), and likely Arg(163). Northern blot analysis revealed the presence of a truncated secM transcript, likely issued from a secM-secA cleavage. The level of secM transcripts was decreased either when secM translation was totally prevented or when Pro(166) was mutated. However, the accumulation of a truncated secM transcript required secM translation and was prevented when the SecM arrest sequence was inactivated by a point mutation changing Pro(166) to Ala. We suggest that ribosome pausing at the site encoding the arrest sequence is required for formation of the truncated secM mRNA. SsrA tagging affected neither the presence of the secM mRNA nor secA expression, even under translocation-defective conditions. It is therefore likely that SsrA tagging of SecM occurs only after cleavage of secM-secA mRNA within the secM open reading frame encoding the SecM arrest sequence. Accumulation of transcripts expressing arrested SecM generated growth inhibition that was alleviated by the SsrA tagging system. Therefore, SsrA tagging of SecM would rescue ribosomes to avoid excessive jamming of the translation apparatus on stop-less secM mRNA.

Two nonredundant SecA homologues function in mycobacteria

The proper extracytoplasmic localization of proteins is an important aspect of mycobacterial physiology and the pathogenesis of Mycobacterium tuberculosis. The protein export systems of mycobacteria have remained unexplored. The Sec-dependent protein export pathway has been well characterized in Escherichia coli and is responsible for transport across the cytoplasmic membrane of proteins containing signal sequences at their amino termini. SecA is a central component of this pathway, and it is highly conserved throughout bacteria. Here we report on an unusual property of mycobacterial protein export--the presence of two homologues of SecA (SecA1 and SecA2). Using an allelic-exchange strategy in Mycobacterium smegmatis, we demonstrate that secA1 is an essential gene. In contrast, secA2 can be deleted and is the first example of a nonessential secA homologue. The essential nature of secA1, which is consistent with the conserved Sec pathway, leads us to believe that secA1 represents the equivalent of E. coli secA. The results of a phenotypic analysis of a Delta secA2 mutant of M. smegmatis are presented here and also indicate a role for SecA2 in protein export. Based on our study, it appears that SecA2 can assist SecA1 in the export of some proteins via the Sec pathway. However, SecA2 is not the functional equivalent of SecA1. This finding, in combination with the fact that SecA2 is highly conserved throughout mycobacteria, suggests a second role for SecA2. The possibility exists that another role for SecA2 is to export a specific subset of proteins.

The N-terminal region of the Oenococcus oeni bacteriophage fOg44 lysin behaves as a bona fide signal peptide in Escherichia coli and as a cis-inhibitory element, preventing lytic activity on oenococcal cells

The function of the N-terminal region of the Oenococcus oeni phage fOg44 lysin (Lys44) as an export signal was investigated. We observed that when induced in Escherichia coli, Lys44 was cleaved between residues 27 and 28 in a SecA-dependent manner. Lys44 processing could be blocked by a specific signal peptidase inhibitor and was severely reduced by modification of the cleavage site. The lethal effect of Lys44 expression observed in E. coli was ascribed to the presence of its N-terminal 27-residue sequence, as its deletion resulted in the production of a nontoxic, albeit active, product. We have further established that lytic activity in oenococcal cells was dependent on Lys44 processing. An active protein with the molecular mass expected for the cleaved enzyme was detected in extracts from O. oeni-infected cells. The temporal pattern of its appearance suggests that synthesis and export of Lys44 in the infected host progress along with phage maturation. Overall, these results provide, for the first time, experimental evidence for the presence of a signal peptide in a bacteriophage lysin. Database searches and alignment of protein sequences support the prediction that other known O. oeni and Lactococcus lactis phages also encode secretory lysins. The evolutionary significance of a putative phage lysis mechanism relying on secretory lytic enzymes is tentatively discussed, on the basis of host cell wall structure and autolytic capacity.

Nucleotide binding activity of SecA homodimer is conformationally regulated by temperature and altered by prlD and azi mutations

SecA ATPase is critical for protein translocation across the Escherichia coli inner membrane. To understand this activity further, the high affinity nucleotide binding activity of SecA was characterized. We found that at 4 degrees C SecA homodimer binds one ADP molecule with high affinity. This nucleotide binding activity was conformationally regulated by temperature: at low temperature SecA affinity for ADP was high with a slow exchange rate, whereas at high temperature the converse was true. Azi- and PrlD-SecA proteins that confer azide-resistant and signal sequence suppressor phenotypes were found to have reduced affinity for ADP and accelerated exchange rates compared with wild type SecA. Consistent with this observation, fluorescence and proteolysis studies indicated that these proteins had a conformationally relaxed state at a reduced temperature compared with SecA. The level of Azi- and PrlD-SecA protein was also elevated in inverted membrane vesicles where it displayed higher membrane ATPase activity. These results provide the first direct evidence for conformational regulation of the SecA-dependent nucleotide cycle, its alteration in azi and prlD mutants, and its relevance to in vivo protein export.

The efficient export of NADP-containing glucose-fructose oxidoreductase to the periplasm of Zymomonas mobilis depends both on an intact twin-arginine motif in the signal peptide and on the generation of a structural export signal induced by cofactor binding

The periplasmic, NADP-containing glucose-fructose oxidoreductase of the gram-negative bacterium Zymomonas mobilis belongs to a class of redox cofactor-dependent enzymes which are exported with the aid of a signal peptide containing a so-called twin-arginine motif. In this paper we show that the replacement of one or both arginine residues results in drastically reduced translocation of glucose-fructose oxidoreductase to the periplasm, showing that this motif is essential. Mutant proteins which, in contrast to wild-type glucose-fructose oxidoreductase, bind NADP in a looser and dissociable manner, were severely affected in the kinetics of plasma membrane translocation. These results strongly suggest that the translocation of glucose-fructose oxidoreductase into the periplasm uses a Sec-independent apparatus which recognizes, as an additional signal, a conformational change in the structure of the protein, most likely triggered by cofactor binding. Furthermore, these results suggest that glucose-fructose oxidoreductase is exported in a folded form. A glucose-fructose oxidoreductase:beta-galactosidase fusion protein is not lethal to Z. mobilis cells and leads to the accumulation of the cytosolic preform of wild-type glucose-fructose oxidoreductase expressed in trans but not of a typical Sec-substrate (OmpA), indicating that the glucose-fructose oxidoreductase translocation apparatus can be blocked without interfering with the export of essential proteins via the Sec pathway.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Differential dependence of levansucrase and alpha-amylase secretion on SecA (Div) during the exponential phase of growth of Bacillus subtilis

SecA, the translocation ATPase of the preprotein translocase, accounts for 0.25% of the total protein in a degU32(Hy) Bacillus subtilis strain in logarithmic phase. The SecA level remained constant irrespective of the demand for exoprotein production but dropped about 12-fold during the late stationary phase. Modulation of the level of functional SecA during the exponential phase of growth affected differently the secretion of levansucrase and alpha-amylase overexpressed under the control of the sacB leader region. The level of SecA was reduced in the presence of sodium azide and in the div341 thermosensitive mutant at nonpermissive temperatures. Overproduction of SecA was obtained with a multicopy plasmid bearing secA. The gradual decrease of the SecA level reduced the yield of secreted levansucrase with a concomitant accumulation of unprocessed precursor in the cells, while an increase in the SecA level resulted in an elevation of the production of exocellular levansucrase. In contrast, alpha-amylase secretion was almost unaffected by high concentrations of sodium azide or by very low levels of SecA. Secretion defects were apparent only under conditions of strong SecA deprivation of the cell. These data demonstrate that the alpha-amylase and levansucrase precursors markedly differ in their dependency on SecA for secretion. It is suggested that these precursors differ in their binding affinities for SecA.

secG and temperature modulate expression of azide-resistant and signal sequence suppressor phenotypes of Escherichia coli secA mutants

SecA is a dynamic protein that undergoes ATP-dependent membrane cycling to drive protein translocation across the Escherichia coli inner membrane. To understand more about this process, azide-resistant (azi) and signal sequence suppressor (prlD) alleles of secA were studied. We found that azide resistance is cold sensitive because of a direct effect on protein export, suggesting that SecA-membrane interaction is regulated by an endothermic step that is azide inhibitable. secG function is required for expression of azide-resistant and signal sequence suppressor activities of azi and prlD alleles, and in turn, these alleles suppress cold-sensitive and export-defective phenotypes of a secG null mutant. These remarkable genetic observations support biochemical data indicating that SecG promotes SecA membrane cycling and that this process is dependent on an endothermic change in SecA conformation.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Expression of the Zymomonas mobilis gfo gene or NADP-containing glucose:fructose oxidoreductase (GFOR) in Escherichia coli. Formation of enzymatically active preGFOR but lack of processing into a stable periplasmic protein

Glucose:fructose oxidoreductase (GFOR) of the gram-negative bacterium Zymomonas mobilis is a periplasmic enzyme with tightly bound cofactor NADP. The preprotein carries an unusually long N-terminal signal peptide of 52 amino acid residues. Expression of the gfo gene in cells of Escherichia coli K12, under the control of a tac promoter, led to immunologically detectable proteins in western blots, and to the formation of an enzymatically active precursor form (preGFOR), located in the cytosol. Processing of preGFOR to the mature form was not observed in E. coli. Replacement of the authentic GFOR signal peptide by the shorter signal peptides of PhoA or OmpA from E. coli led to processing of the respective GFOR precursor proteins. However, the processed proteins were unstable and rapidly degraded in the periplasm unless an E. coli mutant was used that carried a triple lesion for periplasmic and outer-membrane proteases. When fusion-protein export was inhibited by sodium azide or carboxylcyanide m-chlorophenylhydrazone, the cytoplasmic precursor forms of the respective preGFOR were not degraded. A major protease-resistant GFOR peptide from the OmpA-GFOR fusion was found within spheroplasts of E. coli to which NADP had been added externally. The formation of this peptide did not occur in the presence of NAD. It is concluded that NADP is required for GFOR to fold into its native conformation and that its absence from the E. coli periplasm is responsible for failure to form a stable periplasmic protein. The results strongly suggest that, in Z. mobilis, additional protein factors are required for the transport of NADP across the plasma membrane and/or incorporation of NADP into the GFOR apoenzyme.

Protein secretion in streptomycetes

Some aspects of the current knowledge on protein secretion in streptomycetes are presented including recent data on the identification of genes in the general secretory pathway, on the importance of the signal peptide structure and on the number of ribosome-binding sites inside signal peptides which can influence the production level of a gene product.

Production of sodium azide inactivating substance by resistant mutants of Salmonella typhimurium and other bacteria

We have found that azide-resistant mutants of Salmonella typhimurium and of other bacteria studied produce a substance which inactivates the azide. The production of this substance was proved by the demonstration of a satellite growth of azide-sensitive cells around colonies of azide-resistant mutants and by testing azide inactivating properties of culture filtrates of the azide-resistant strains. The same substance was found to be present in lower concentrations in culture filtrates of wild-type sensitive strains. In both cultures of sensitive strains, it was apparently produced by the resistant mutants and not by the sensitive cells. The substance does not pass across a dialysis membrane and is heat stable. It has a high molecular weight but is not a protein.

Cloning of a secA homolog from Streptomyces lividans 1326 and overexpression in both S. lividans and Escherichia coli

We cloned a gene encoding a SecA homolog from Streptomyces lividans 1326, a Gram-positive bacterium known to produce large amounts of extracellular proteins. A protein sequence alignment with the other bacterial SecA homologs revealed that S. lividans SecA shares from 39.5 to 44% identity with them, while it shares 34.2 to 37.2% identity with SecA homologs from plastids of algae and plants. We overexpressed the secA gene in S. lividans 1326 and Escherichia coli MM52 and in both cases we observed the production of a protein with an apparent molecular mass of 117.4 kDa. Although S. lividans SecA is similar to E. coli SecA, it does not complement a thermosensitive mutation in the E. coli secA gene. However, a hybrid polypeptide consisting of the N-terminal portion (first 242 amino acids) of the S. lividans SecA and the C-terminal portion (657 a.a.) of the wild-type E. coli SecA was able to complement this mutant.

Two-step secretion of the Serratia marcescens extracellular nuclease

The extracellular nuclease of Serratia marcescens is one of a wide variety of enzymes secreted into the growth medium. Its appearance occurs late in the growth of a culture, and its gene, nucA, is transcriptionally regulated in a complex fashion by growth phase and other factors. Pulse-labeling studies reveal that extracellular secretion of nuclease occurs as a two-step process. In the first step, nuclease is rapidly translocated across the cytoplasmic membrane into the periplasm, where it accumulates as a mature active nuclease. A precursor protein, nuclease still carrying its signal sequence, was detected in the presence of carbonyl cyanide m-chlorophenylhydrazone or sodium azide, suggesting that this initial translocation and signal processing step involves an energy-dependent and Sec-dependent pathway in S. marcescens. The second step of secretion across the outer membrane is a slow process requiring between 30 to 120 min, depending on growth conditions.

Suppression of signal sequence defects and azide resistance in Escherichia coli commonly result from the same mutations in secA

The SecA protein of Escherichia coli is required for protein translocation from the cytoplasm. The complexity of SecA function is reflected by missense mutations in the secA gene that confer several different phenotypes: (i) conditional-lethal alleles cause a generalized block in protein secretion, resulting in the cytoplasmic accumulation of the precursor forms of secreted proteins; (ii) azi alleles confer resistance to azide at concentrations up to 4 mM; and (iii) prlD alleles suppress a number of signal sequence mutations in several different genes. To gain further insights into the role of SecA in protein secretion, we have isolated and characterized a large number of prlD mutations, reasoning that these mutations alter a normal function of wild-type SecA. Our results reveal a striking coincidence of signal sequence suppression and azide resistance: the majority of prlD alleles also confer azide resistance, and all azi alleles tested are suppressors. We suggest that this correlation reflects the mechanism(s) of signal sequence suppression. There are two particularly interesting subclasses of prlD and azi alleles. First, four of the prlD and azi alleles exhibit special properties: (i) as suppressors they are potent enough to allow PrlD (SecA) inactivation by a toxic LacZ fusion protein marked with a signal sequence mutation (suppressor-directed inactivation), (ii) they confer azide resistance, and (iii) they cause modest defects in the secretion of wild-type proteins. Sequence analysis reveals that all four of these alleles alter Tyr-134 in SecA, changing it to Ser, Cys, or Asn. The second subclass consists of seven prlD alleles that confer azide supersensitivity, and sequence analysis reveals that six of these alleles are changes of Ala-507 to Val. Both of the affected amino acids are located within different putative ATP-binding regions of SecA and thus may affect ATPase activities of SecA. We suggest that the four azide-resistant mutations slow an ATPase activity of SecA, thus allowing successful translocation of increased amounts of mutant precursor proteins.

Isolation and characterization of a Bacillus subtilis secA mutant allele conferring resistance to sodium azide

A mutation has been isolated in the Bacillus subtilis secA gene (secA10) which allows cell growth and residual protein translocation in the presence of 1.5 mM sodium azide. Besides conferring resistance to sodium azide, the corresponding SecA10 mutant protein, in which glutamic acid at position 338 has been changed to glycine, seems to possess a secretion defect even in the absence of azide. In addition, the secA10 mutant protein was found to be recessive to wild-type secA with regard to azide resistance. Our results strongly suggest that, like the situation in Escherichia coli, the B. subtilis SecA protein is a main target for the lethal action of sodium azide.

Transport of haemin across the cytoplasmic membrane through a haemin-specific periplasmic binding-protein-dependent transport system in Yersinia enterocolitica

The Yersinia enterocolitica O:8 periplasmic binding-protein-dependent transport (PBT) system for haemin was cloned and characterized. It consisted of four proteins: the periplasmic haemin-binding protein HemT, the haemin permease protein HemU, the ATP-binding hydrophilic protein HemV and the putative haemin-degrading protein HemS. Y. enterocolitica strains mutated in hemU or hemV genes were unable to use haemin as an iron source whereas those mutated in the hemT gene were able to use haemin as an iron source. As Escherichia coli strains expressing only the haemin outer membrane receptor protein HemR from Y. enterocolitica were capable of using haemin as an iron source the existence of an E. coli K-12 haemin-specific PBT system is postulated. The first gene in the Y. enterocolitica haemin-specific PBT system encoded a protein, HemS, which is probably involved in the degradation of haemin in the cytoplasm. The presence of the hemS gene was necessary to prevent haemin toxicity in E. coli strains that accumulate large amounts of haemin in the cytoplasm. We propose a model of haemin utilization in Y. enterocolitica in which HemT, HemU and HemV proteins transport haemin into the cytoplasm where it is degraded by HemS thereby liberating the iron.

Azide-resistant mutants in Acinetobacter calcoaceticus A2 are defective in protein secretion

Azide, an inhibitor of ATPase, and a specific inhibitor of protein export was used in order to select for protein secretion mutants in Acinetobacter calcoaceticus A2. Two such mutants were isolated that were azide-resistant and defective in the general protein transport system. The mutation also conferred additional phenotypic changes, including an inability to grow on minimal media or at 40 degrees C. The existence of protein secretion mutants with a selectable phenotype may be useful for the genetic study of protein export.

Two distinct ATP-binding domains are needed to promote protein export by Escherichia coli SecA ATPase

Six putative ATP-binding motifs of SecA protein were altered by oligonucleotide-directed mutagenesis to try to define the ATP-binding regions of this multifunctional protein. The effects of the mutations were analysed by genetic and biochemical assays. The results show that SecA contains two essential ATP-binding domains. One domain is responsible for high-affinity ATP binding and contains motifs A0 and B0, located at amino acid residues 102-109 and 198-210, respectively. A second domain is responsible for low-affinity ATP binding and contains motifs A3 and a predicted B motif located at amino acid residues 503-511 and 631-653, respectively. The ATP-binding properties of both domains were essential for SecA-dependent translocation ATPase and in vitro protein translocation activities. The significance of these findings for the mechanism of SecA-dependent protein translocation is discussed.

An outer membrane protein (OmpA) of Escherichia coli can be translocated across the cytoplasmic membrane of Bacillus subtilis

The translocation of secretory proteins derived from a Gram-positive (Staphylococcus hyicus prolipase) or a Gram-negative (Escherichia coli pre-OmpA protein) bacterium across the cytoplasmic membrane was studied in E. coli and Bacillus subtilis. In both microorganisms, the prolipase was found to be secreted across the plasma membrane when either the pre-prolipase signal peptide (38 amino acids in length) or the pre-OmpA signal peptide (21 amino acids in length) was used. Expression of the gene encoding the authentic pre-OmpA protein in B. subtilis resulted in the translocation of mature OmpA protein across the plasma membrane. Processing of the OmpA precursor in B. subtilis required the electrochemical potential and was sensitive to sodium azide, suggesting that the B. subtilis SecA homologue was involved in the translocation process. The mature OmpA protein, which was most likely present in an aggregated state, was fully accessible to proteases in protoplasted cells. Therefore, our results clearly demonstrate that an outer membrane protein can be secreted by B. subtilis, supporting the notion that the basic mechanism of protein translocation is highly conserved in Gram-positive and Gram-negative bacteria.

The complete general secretory pathway in gram-negative bacteria

The unifying feature of all proteins that are transported out of the cytoplasm of gram-negative bacteria by the general secretory pathway (GSP) is the presence of a long stretch of predominantly hydrophobic amino acids, the signal sequence. The interaction between signal sequence-bearing proteins and the cytoplasmic membrane may be a spontaneous event driven by the electrochemical energy potential across the cytoplasmic membrane, leading to membrane integration. The translocation of large, hydrophilic polypeptide segments to the periplasmic side of this membrane almost always requires at least six different proteins encoded by the sec genes and is dependent on both ATP hydrolysis and the electrochemical energy potential. Signal peptidases process precursors with a single, amino-terminal signal sequence, allowing them to be released into the periplasm, where they may remain or whence they may be inserted into the outer membrane. Selected proteins may also be transported across this membrane for assembly into cell surface appendages or for release into the extracellular medium. Many bacteria secrete a variety of structurally different proteins by a common pathway, referred to here as the main terminal branch of the GSP. This recently discovered branch pathway comprises at least 14 gene products. Other, simpler terminal branches of the GSP are also used by gram-negative bacteria to secrete a more limited range of extracellular proteins.

Ferrioxamine uptake in Yersinia enterocolitica: characterization of the receptor protein FoxA

The gene for the high-affinity outer membrane ferrioxamine receptor FoxA of Yersinia enterocolitica was cloned in Escherichia coli K-12. A foxA mutant of Yersinia could be complemented by the cloned DNA fragment. The FoxA encoding region was sequenced and an open reading frame encoding 710 amino acids, including a signal sequence of 26 amino acids, was deduced. The mature FoxA protein consisted of 684 amino acids and had a molecular mass of 75,768 Da. FoxA shared 33% amino acid sequence homology with FhuA, the ferrichrome receptor of Escherichia coli. Based on the homologies with FhuA and other TonB-dependent receptors a topological model of FoxA is presented.

A lipoprotein of Yersinia enterocolitica facilitates ferrioxamine uptake in Escherichia coli

A cloned fragment of Yersinia enterocolitica DNA complemented the defect in ferrioxamine B uptake of an Escherichia coli fhuE mutant lacking the outer membrane high-affinity transport protein FhuE. Subcloning revealed that a 13.7-kDa outer membrane protein was required for complementation. The amino acid sequence deduced from the nucleotide sequence showed extensive homology to PCPHi, an outer membrane lipoprotein of Haemophilus influenzae. We therefore termed this protein PCPYe. Plasmid-encoded pcpY mediated a low-affinity uptake of ferrioxamine B which may be caused by changes in the permeability of the outer membrane due to an overexpression of this outer membrane protein. A transposon insertion mutant in the plasmid-encoded pcpY gene was transferred into the chromosome of Y. enterocolitica. The resulting mutation had no effect on the high-affinity uptake of ferrioxamine B in Yersinia cells. Using the antibiotic ferrimycin we were able to isolate a Y. enterocolitica mutant lacking the high-affinity outer membrane receptor for ferrioxamine uptake, termed FoxA.

Pore-forming bacterial protein hemolysins (cytolysins)

Protein toxins forming pores in biological membranes occur frequently in Gram-positive and Gram-negative bacteria. They kill either bacteria or eukaryotic cells (at most, a few seem to act on both groups of organisms). Most of the toxins affecting eukaryotes have clearly been shown to be related to the pathogenicity of the producing organisms. Toxin formation frequently involves a number of genes which encode the toxin polypeptide as well as proteins for toxin activation and secretion. Regulation of toxin production is usually coupled with that of the synthesis of a number of other virulence factors. Iron is the only known environmental factor that regulates transcription of a number of toxin genes by a Fur repressor-type mechanism, as has been originally described in Escherichia coli. Interestingly, the thiol-activated hemolysins (cytolysins) of Gram-positive bacteria contain a single cysteine which can be replaced by alanine without affecting the cytolytic activity. The Gram-negative hemolysins (cytolysins) are usually synthesized as precursor proteins, then covalently modified to yield an active hemolysin and secreted via specific export systems, which differ for various types of hemolysins.