Identification of five new essential genes involved in the synthesis of a secreted protein in Escherichia coli

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.

The way is the goal: how SecA transports proteins across the cytoplasmic membrane in bacteria

In bacteria, translocation of most soluble secreted proteins (and outer membrane proteins in Gram-negative bacteria) across the cytoplasmic membrane by the Sec machinery is mediated by the essential ATPase SecA. At its core, this machinery consists of SecA and the integral membrane proteins SecYEG, which form a protein conducting channel in the membrane. Proteins are recognised by the Sec machinery by virtue of an internally encoded targeting signal, which usually takes the form of an N-terminal signal sequence. In addition, substrate proteins must be maintained in an unfolded conformation in the cytoplasm, prior to translocation, in order to be competent for translocation through SecYEG. Recognition of substrate proteins occurs via SecA—either through direct recognition by SecA or through secondary recognition by a molecular chaperone that delivers proteins to SecA. Substrate proteins are then screened for the presence of a functional signal sequence by SecYEG. Proteins with functional signal sequences are translocated across the membrane in an ATP-dependent fashion. The current research investigating each of these steps is reviewed here.

A putative cro-like repressor contributes to arylomycin resistance in Staphylococcus aureus

Antibiotic-resistant bacteria are a significant public health concern and motivate efforts to develop new classes of antibiotics. One such class of antibiotics is the arylomycins, which target type I signal peptidase (SPase), the enzyme responsible for the release of secreted proteins from their N-terminal leader sequences. Despite the essentiality, conservation, and relative accessibility of SPase, the activity of the arylomycins is limited against some bacteria, including the important human pathogen Staphylococcus aureus. To understand the origins of the limited activity against S. aureus, we characterized the susceptibility of a panel of strains to two arylomycin derivatives, arylomycin A-C16 and its more potent analog arylomycin M131. We observed a wide range of susceptibilities to the two arylomycins and found that resistant strains were sensitized by cotreatment with tunicamycin, which inhibits the first step of wall teichoic acid synthesis. To further understand how S. aureus responds to the arylomycins, we profiled the transcriptional response of S. aureus NCTC 8325 to growth-inhibitory concentrations of arylomycin M131 and found that it upregulates the cell wall stress stimulon (CWSS) and an operon consisting of a putative transcriptional regulator and three hypothetical proteins. Interestingly, we found that mutations in the putative transcriptional regulator are correlated with resistance, and selection for resistance ex vivo demonstrated that mutations in this gene are sufficient for resistance. The results begin to elucidate how S. aureus copes with secretion stress and how it evolves resistance to the inhibition of SPase.

Combined inactivation and expression strategy to study gene function under physiological conditions: application to identification of new Escherichia coli adhesins

In bacteria, whereas disruption methods have been improved recently, the use of plasmid complementation strategies are still subject to limitations, such as cloning difficulties, nonphysiological levels of gene expression, or a requirement for antibiotics as plasmid selection pressure. Moreover, because of the pleiotropic modifications of cell physiology often induced by plasmid-based complementation, these strategies may introduce biases when biological process such as adhesion or biofilm formation are studied. We developed a plasmid-free approach that combines the lambda-red linear DNA recombination method with site-directed insertion of a repression and expression (RExBAD) cassette which places a functional pBAD promoter upstream of a target gene. We showed that this method permits both inactivation and modulation of most Escherichia coli gene expression, including expression of toxin and essential genes. We used this strategy to study adhesion and bacterial biofilms by placing the RExBAD cassette in front of the tra operon, encoding the DNA transfer and pilus genes on the F conjugative plasmid, and in front of flu, the antigen 43 (Ag43) autotransporter adhesin-encoding gene. In silico analysis revealed the existence of 10 genes with homology to the Ag43 gene that were good candidates for genes that encode putative new adhesins in E. coli. We used the RExBAD strategy to study these genes and demonstrated that induction of expression of four of them is associated with adhesion of E. coli to abiotic surfaces. The potential use of the RExBAD approach to study the function of cryptic or uncharacterized genes in large-scale postgenomic functional analyses is discussed.

Multifaceted physiological response allows yeast to adapt to the loss of the signal recognition particle-dependent protein-targeting pathway

Translational control has recently been recognized as an important facet of adaptive responses to various stress conditions. We describe the adaptation response of the yeast Saccharomyces cerevisiae to the loss of one of two mechanisms to target proteins to the secretory pathway. Using inducible mutants that block the signal recognition particle (SRP) pathway, we find that cells demonstrate a physiological response to the loss of the SRP pathway that includes specific changes in global gene expression. Upon inducing the loss of the SRP pathway, SRP-dependent protein translocation is initially blocked, and cell growth is considerably slowed. Concomitantly, gene expression changes include the induction of heat shock genes and the repression of protein synthesis genes. Remarkably, within hours, the efficiency of protein sorting improves while cell growth remains slow in agreement with the persistent repression of protein synthesis genes. Our results suggest that heat shock gene induction serves to protect cells from mislocalized precursor proteins in the cytosol, whereas reduced protein synthesis helps to regain efficiency in protein sorting by reducing the load on the protein translocation apparatus. Thus, we suggest that cells trade speed in cell growth for fidelity in protein sorting to adjust to life without SRP.

Chaperone-like activities of the CsaA protein of Bacillus subtilis

The growth and protein export defects of Escherichia coli secA51(Ts) strains can be suppressed by the CsaA protein of Bacillus subtilis. The present studies indicate that this effect can be attributed to chaperone-like activities of CsaA. First, CsaA stimulated protein export in secB, groES and dnaJ mutant strains of E. coli. Second, CsaA suppressed the growth defects of dnaK, dnaJ and grpE mutants of E. coli. Third, and most importantly, CsaA exhibited chaperone-like properties by stimulating the reactivation of heat-denatured firefly luciferase in groEL, groES, dnaK and grpE mutant strains of E. coli, and by preventing the aggregation of heat-denatured luciferase in vitro. Thus, it seems that CsaA suppresses the growth and secretion defects of E. coli secA(Ts) strains either by improving the translocation competence of exported pre-proteins, thereby making them better substrates for mutant SecA proteins, or by stimulating the translocation activity of mutant SecA proteins.

Translocation, folding, and stability of the HflKC complex with signal anchor topogenic sequences

HflK and HflC are plasma membrane proteins of Escherichia coli, each having a large C-terminal domain exposed to the periplasmic space and an N-terminally located transmembrane segment, which should act as a signal anchor sequence for their biogenesis. They form a complex, HflKC. We studied in vivo processes of biogenesis of this pair of membrane proteins. Translocation of the C-terminal domains across the membrane, as assessed by their accessibility to externally added protease, was completed within 1 min after the synthesis in wild-type cells as well as in the secB mutant cells or in the FtsY-depleted cells. In contrast, translocation of these domains was retarded markedly when sodium azide was added to inhibit SecA ATPase and blocked almost completely in secY- or secD-defective mutant cells. Thus, although targeting of these membrane proteins depends neither on the SecB chaperone nor on the SRP pathway, their translocation occurs exclusively via the Sec translocase complex. Translocated HflK molecules were then folded into a partially protease-resistant conformation, taking a few minutes, and this folding was induced upon association with HflC. Singly expressed HflK and HflC were unstable in vivo and periplasmic proteases DegP and Prc were involved in the degradation of the HflK subunit. We characterized several hflA alleles isolated in early studies; they alter the HflK or the HflC sequence and destabilize the HflKC complex.

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.

Identification and characterization of a newly isolated shiga toxin 2-converting phage from shiga toxin-producing Escherichia coli

Shiga toxins 1 (Stx1) and 2 (Stx2) are encoded by toxin-converting bacteriophages of Stx-producing Escherichia coli (STEC), and so far two Stx1- and one Stx2-converting phages have been isolated from two STEC strains (A. D. O'Brien, J. W. Newlands, S. F. Miller, R. K. Holmes, H. W. Smith, and S. B. Formal, Science 226:694-696, 1984). In this study, we isolated two Stx2-converting phages, designated Stx2Phi-I and Stx2Phi-II, from two clinical strains of STEC associated with the outbreaks in Japan in 1996 and found that Stx2Phi-I resembled 933W, the previously reported Stx2-converting phage, in its infective properties for E. coli K-12 strain C600 while Stx2Phi-II was distinct from them. The sizes of the plaques of Stx2Phi-I and Stx2Phi-II in C600 were different; the former was larger than the latter. The restriction maps of Stx2Phi-I and Stx2Phi-II were not identical; rather, Stx2Phi-II DNA was approximately 3 kb larger than Stx2Phi-I DNA. Furthermore, Stx2Phi-I and Stx2Phi-II showed different phage immunity, with Stx2Phi-I and 933W belonging to the same group. Infection of C600 by Stx2Phi-I or 933W was affected by environmental osmolarity differently from that by Stx2Phi-II. When C600 was grown under conditions of high osmolarity, the infectivity of Stx2Phi-I and 933W was greatly decreased compared with that of Stx2Phi-II. Examination of the plating efficiency of the three phages for the defined mutations in C600 revealed that the efficiency of Stx2Phi-I and 933W for the fadL mutant decreased to less than 10(-7) compared with that for C600 whereas the efficiency of Stx2Phi-II decreased to 0.1% of that for C600. In contrast, while the plating efficiency of Stx2Phi-II for the lamB mutant decreased to a low level (0.05% of that for C600), the efficiencies of Stx2Phi-I and 933W were not changed. This was confirmed by the phage neutralization experiments with isolated outer membrane fractions from C600, fadL mutant, or lamB mutant or the purified His6-tagged FadL and LamB proteins. Based on the data, we concluded that FadL acts as the receptor for Stx2Phi-I and Stx2Phi-II whereas LamB acts as the receptor only for Stx2Phi-II.

Multicopy suppression of cold-sensitive sec mutations in Escherichia coli

Mutations in the secretory (sec) genes in Escherichia coli compromise protein translocation across the inner membrane and often confer conditional-lethal phenotypes. We have found that overproduction of the chaperonins GroES and GroEL from a multicopy plasmid suppresses a wide array of cold-sensitive sec mutations in E. coli. Suppression is accompanied by a stimulation of precursor protein translocation. This multicopy suppression does not bypass the Sec pathway because a deletion of secE is not suppressed under these conditions. Surprisingly, progressive deletion of the groE operon does not completely abolish the ability to suppress, indicating that the multicopy suppression of cold-sensitive sec mutations is not dependent on a functional groE operon. Indeed, overproduction of proteins unrelated to the process of protein export suppresses the secE501 cold-sensitive mutation, suggesting that protein overproduction, in and of itself, can confer mutations which compromise protein synthesis and the observation that low levels of protein synthesis inhibitors can suppress as well. In all cases, the mechanism of suppression is unrelated to the process of protein export. We suggest that the multicopy plasmids also suppress the sec mutations by compromising protein synthesis.

ssaD1, a suppressor of secA51(Ts) that renders growth of Escherichia coli cold sensitive, is an early amber mutation in the transcription factor gene nusB

Complementation analysis of the ssaD1 mutation, isolated as a suppressor of the secA51(Ts) mutation that renders growth of Escherichia coli cold sensitive, was used to show that ssaD corresponds to nusB, a gene known to be important in transcription antitermination. DNA sequence analysis of the ssaD1 allele showed that it creates an amber mutation in the 15th codon of nusB. Analysis of the effect of different levels of NusB protein on secA transcription and translation suggested that NusB plays little or no role in the control of secA expression. Accordingly, mechanisms by which nusB inactivation can lead to suppression of secA51(Ts) and secY24(Ts) mutations without affecting secA expression need to be considered.

Functions of the gene products of Escherichia coli

A list of currently identified gene products of Escherichia coli is given, together with a bibliography that provides pointers to the literature on each gene product. A scheme to categorize cellular functions is used to classify the gene products of E. coli so far identified. A count shows that the numbers of genes concerned with small-molecule metabolism are on the same order as the numbers concerned with macromolecule biosynthesis and degradation. One large category is the category of tRNAs and their synthetases. Another is the category of transport elements. The categories of cell structure and cellular processes other than metabolism are smaller. Other subjects discussed are the occurrence in the E. coli genome of redundant pairs and groups of genes of identical or closely similar function, as well as variation in the degree of density of genetic information in different parts of the genome.

Suppression of the growth and export defects of an Escherichia coli secA(Ts) mutant by a gene cloned from Bacillus subtilis

A gene library of Bacillus subtilis chromosomal DNA was screened for genes capable of reverting the growth defects of the Escherichia coli secA51(Ts) mutant at 42 degrees C. A B. subtilis gene, designated csaA, was found to phenotypically suppress not only the growth defects of the E. coli mutant, but also to relieve the detrimental accumulation of precursors of exported proteins. The csaA gene encoded a protein of 15 kDa (137 amino acids) and was likely to be the distalmost member of an operon. No similarity to csaA was found among DNA or protein sequences deposited in databases. In contrast to other homologous or heterologous suppressors of the E. coli secA51(Ts) mutation, the csaA gene did not exert pleiotropic effects on either the E. coli secY24(Ts) or lep9(Ts) mutations. However, it restored the ability of a SecB-deficient mutant to grow on complex medium. It is proposed that CsaA serves as a molecular chaperone for exported proteins or alternatively acts by stabilizing the SecA protein.

Multicopy suppression: an approach to understanding intracellular functioning of the protein export system

Escherichia coli genes were cloned onto a multicopy plasmid and selected by the ability to restore growth and protein export defects caused by a temperature-sensitive secY or secA mutation. When secA51 was used as the primary mutation, only clones carrying groE, which specifies the chaperonin class of heat shock protein, were obtained. Selection using secY24 yielded three major classes of genes. The first class encodes another heat shock protein, HtpG; the most frequently obtained second class encodes a neutral histonelike protein, H-NS; and the third class, msyB, encodes a 124-residue protein of which 38 residues are acidic amino acids. Possible mechanisms of suppression as well as the significance and limitations of the multicopy suppression approach are discussed.

Escherichia coli sec mutants accumulate a processed immature form of maltose-binding protein (MBP), a late-phase intermediate in MBP export

Protein translocation across the Escherichia coli cytoplasmic membrane may consist of several temporally or topographically distinct steps. Although early events in the translocation pathway have been characterized to some extent, the mechanisms responsible for the trans-bilayer movement of a polypeptide are only poorly understood. This article reports on our attempts to dissect the translocation pathway in vivo. A processed form of maltose-binding protein (MBP) was detected in the spheroplasts of secY and secA temperature-sensitive mutant cells that had been pulse-labeled at the permissive temperature (30 degrees C). This species of molecule was found to have an electrophoretic mobility identical to that of the mature MBP, but a considerable fraction of it was inaccessible to externally added protease. It had not attained the protease-resistant conformation characteristically observed for the exported mature protein. The radioactivity associated with this species decreased during chase and was presumably converted into the exported mature form, a process that required energy, probably the proton motive force, as demonstrated by its inhibition by an energy uncoupler. The spheroplast-associated processed form was more predominantly observed in the presence of a low concentration of chloramphenicol. A similar intermediate was also detected for beta-lactamase in wild-type cells. These results suggest that in a late phase of translocation, the bulk of the polypeptide chain can move through the membrane in the absence of the covalently attached leader peptide, and the secA-secY gene products are somehow involved in this process. We termed the processed intermediates processed immature forms.

The sec and prl genes of Escherichia coli

Two general approaches have been used to define genetically the genes that encode components of the cellular protein export machinery. One of these strategies identifies mutations that confer a conditional-lethal, pleiotropic export defect (sec, secretion). The other identifies dominant suppressors of signal sequence mutations (prl, protein localization). Subsequent characterization reveals that in at least three cases, prlA/secY, prlD/secA, and prlG/secE, both types of mutations are found within the same structural gene. This convergence is satisfying and provides compelling evidence for direct involvement of these gene products in the export process.

Structure, function, and biogenesis of SecY, an integral membrane protein involved in protein export

The E. coli secY (prlA) gene, located in the operator-distal part of the spc ribosomal protein operon, codes for an integral membrane protein, SecY. The phenotypes of temperature-sensitive and cold-sensitive mutations in secY suggest that the SecY protein plays an essential role in vivo to facilitate protein translocation, whereas the prlA mutations in this gene suggest that SecY may interact with the signal sequence of translocating polypeptides. SecY contains most probably six cytoplasmic and five periplasmic domains, as well as 10 transmembrane segments. Such membrane-embedded structure may confer the SecY protein a "translocator" function, in which it provides a protein-aceous pathway for passage of secreted as well as membrane proteins. Results obtained by in vitro analyses of the translocation reactions, as well as some new phenotypes of the secY mutants, are consistent with this notion. Possible interaction of SecY with other secretion and chaperone-like factors is also discussed.

SecA protein: autoregulated initiator of secretory precursor protein translocation across the E. coli plasma membrane

Several classes of secA mutants have been isolated which reveal the essential role of this gene product for E. coli cell envelope protein secretion. SecA-dependent, in vitro protein translocation systems have been utilized to show that SecA is an essential, plasma membrane-associated, protein translocation factor, and that SecA's ATPase activity appears to play an essential but as yet undefined role in this process. Cell fractionation studies suggested that SecA protein is in a dynamic state within the cell, occurring in soluble, peripheral, and integral membraneous states. These data have been used to argue that SecA is likely to promote the initial insertion of secretory precursor proteins into the plasma membrane in a manner dependent on ATP hydrolysis. The protein secretion capability of the cell has been shown to translationally regulate secA expression with SecA protein serving as an autogenous repressor, although the exact mechanism and purpose of this regulation need to be defined further.

A collection of strains containing genetically linked alternating antibiotic resistance elements for genetic mapping of Escherichia coli

We present a collection of 182 isogenic strains containing genetically linked antibiotic resistance elements located at approximately 1-min intervals around the Escherichia coli chromosome. At most positions both Tn10 (Tetr) and TN10kan (Kanr) elements are available, so that the collection contains a linked set of alternating antibiotic resistance markers. The map position of each insertion has been aligned to the E. coli genetic map as well as to the Kohara ordered clone bank. These strains are designed to be used in a rapid two-step mapping system in E. coli. In the first step, the mutation is localized to a 5- to 15-min region of the chromosome by Hfr mapping with a set of Hfr strains containing either Tn10 or Tn10kan elements located 20 min from their respective origins of transfer. In the second step, the mutation is localized to a 1-min region by P1 transduction, with a collection of isogenic insertion strains as donors. We discuss the uses of this collection of strains to map and eventually to clone a variety of mutations in E. coli.

Construction of an ordered cosmid collection of the Escherichia coli K-12 W3110 chromosome

A cosmid library of the Escherichia coli K-12 W3110 chromosome was constructed in which clones were assigned to locations on the chromosome map by hybridization and genetic marker complementation tests. Approximately 70% of the genome was represented by this library. The identified clones can be maintained in the homologous system and would facilitate genetic studies of E. coli.

Methods for detecting recombinant DNA in the environment

The successful introduction of genetically modified and genetically engineered microorganisms into the environment requires a quantitative evaluation of the survival and dispersion of the microorganisms and specific gene(s) in the environment. The objective of this article is to examine the applicability, suitability, and significance of existing and new methods for detecting and monitoring the recombinant genes or organisms introduced into the environment. Conventional microbiological method(s) involving the selective and differential growth of microorganism(s) adn other quantitative approaches such as the most-probable-number (MPN) method and direct microscopic observation (e.g., acridine orange direct count analysis) have drawbacks and are not specific or universally applicable. Direct enumeration by immunofluorescence by the use of fluorescent dye seems more sensitive although still not perfect. However, the molecular methodologies such as the use of gene probes, plasmid epidemiology, antibiotic resistant marker strains, and protein electrophoresis and bacteriophage sensitivity are receiving more attention. As yet, the technology of DNA:DNA hybridization appears to be very useful, sensitive, and accurate for detecting and monitoring the microorganisms in the environment, although improvements are required. New approaches can be developed which may include biochemical signature compounds as well as gene cassettes to be used in a complementary fashion with conventional and molecular techniques for quantifying specific genotypes and genes in the environment.

Biochemical evidence for the secY24 defect in Escherichia coli protein translocation and its suppression by soluble cytoplasmic factors

The secY (prlA) gene product is an integral membrane protein that has been identified genetically as one of the central components of the Escherichia coli protein translocation machinery. We have examined the effect of the secY24 (temperature-sensitive) mutation on the protein translocation activity of E. coli inverted membrane vesicles. Vesicles isolated from cells carrying this allele and grown at the nonpermissive temperature (42 degrees C) were less than 1% as active in translocation as vesicles isolated from an isogenic secY+ strain under the same conditions. Vesicles from the mutant strain grown at the permissive temperature (32 degrees C) were partially active, but those vesicles preincubated at 40 degrees C lost 90% of their activity. Moreover, the secY24 translocation defect on in vivo- or in vitro-inactivated vesicles was suppressed, or compensated, by an S300 soluble fraction from wild-type cells or from secY24 cells grown at nonpermissive temperature. The suppressing factor(s) was heat-labile and sensitive to proteinase K. These results provide biochemical evidence for the essential role of SecY in the translocation process and indicate that the translocation defect of SecY24 membranes can be compensated for by supplementing with additional soluble cytoplasmic proteins.

secD, a new gene involved in protein export in Escherichia coli

New mutants of Escherichia coli altered in protein export were identified in phoA-lacZ and lamB-lacZ gene fusion strains by searching for mutants that showed an altered lactose phenotype. Several mutations mapped in a new gene, secD. These mutants were, in general, cold sensitive for growth, and the mutations led to an accumulation of precursor of exported proteins. The secD gene is closely linked to tsx on the E. coli chromosome, but separable from another gene proposed to be involved in export, ssaD, which maps nearby. A plasmid carrying secD+ was identified and used to show that the mutations are recessive. The secD gene may code for a component of the cellular export machinery.

Suppression of growth and protein secretion defects in Escherichia coli secA mutants by decreasing protein synthesis

We devised a new selection for conditionally lethal suppressors of secA mutants. This selection allows the isolation of both temperature-sensitive and cold-sensitive suppressor mutations, whereas previous studies were limited to nonlethal or cold-sensitive suppressor mutations. Two temperature-sensitive suppressor mutations lie in genes required for protein synthesis: asnS, the gene for the asparaginyl-tRNA synthetase, and divE, which encodes the tRNASer1. A previously characterized mutation in alaS, the gene for the alanyl-tRNA synthetase, suppresses the growth and secretion defects of a secA mutant. Although the primary effects of these suppressor mutations are different, it is likely that they cause suppression of secA mutations by altering the rate of protein synthesis, since the protein synthesis inhibitors, chloramphenicol and tetracycline, also suppress secA mutations. Chloramphenicol also suppresses the growth defect of certain other sec mutants. We postulate that the impaired secretory capacity of sec mutants can be offset by decreasing the rate of elongation of secreted proteins or by decreasing the total amount of secreted proteins per cell. The results indicate that our initial goal to identify cellular secretory components as suppressors of secA mutations might be difficult to achieve because of a high frequency of nonspecific suppressors that alter protein synthesis. Unexpectedly, the suppressor approach provides a direct genetic selection for mutants in protein synthesis.

Suppressors of the secY24 mutation: identification and characterization of additional ssy genes in Escherichia coli

We previously reported (Shiba et al., J. Bacteriol. 160:696-701, 1984) the isolation and characterization of the mutation (ssy) that suppresses the protein export defect due to the secY24(Ts) mutation and causes cold-sensitive growth of Escherichia coli. This report describes more systematic isolation of ssy mutations. Among temperature-resistant revertants of the secY24 mutant, 65 mutants were found to be cold sensitive. These cold-sensitive mutations have been classified by genetic mapping. Twenty-two mutations fell into the ssyA class previously described. The remaining mutations were located at five new loci: ssyB at 9.5 min between tsx and lon; ssyD around 3 min; ssyE at 72.5 min near secY; ssyF at 20.5 min within rpsA; and ssyG at 69.0 min near argG. Two predominant classes, ssyA and ssyB, are probably affected in protein synthesis at the elongation step, whereas the ssyF mutant contained an altered form of ribosomal protein S1 (the gene product of rpsA). These cold-sensitive ssy mutations which suppress secY24 may define genes whose function is somehow involved in the secY-dependent protein secretion mechanism. However, the existence of multiple suppressor loci makes it unlikely that all of these genes specify additional components of the export machinery. A delicate balance may exist between the systems for synthesizing and exporting proteins.

Single-site chromosomal Tn5 insertions affect the export of pectolytic and cellulolytic enzymes in Erwinia chrysanthemi EC16

Exponentially growing cells of Erwinia chrysanthemi EC16 usually export about 98% of their pectate lyase (PL) and protease, about 40% of their polygalacturonase (PG), and about 60% of their cellulase (endoglucanase or carboxymethyl cellulase; CL). By using the R plasmid, pJB4JI (pPH1JI::Mu::Tn5), three independent Tn5 insertion mutants were obtained that exported normal levels of protease but 10% or less of PL, PG, and CL. Physical analysis revealed that single copies of Tn5 had inserted into the E. chrysanthemi chromosome, producing a similar export-defective (Out-) phenotype. The synthesis of PL, PG, and CL was not affected by the Tn5 insertions. These enzymes were released from the mutants on spheroplast formation, indicating that they were located in the periplasmic space. Tn5 insertions caused the loss of a 35-kilodalton periplasmic protein, but did not alter the outer membrane protein composition. The findings are discussed with respect to the current knowledge on protein export in gram-negative bacteria.

Final steps of the maturation of Omp F, a major protein from the outer membrane of Escherichia coli

Pulse-labelling experiments with E. coli cells allowed us to follow the incorporation of de novo proteins into the outer membrane of the cell envelope. Labelled membrane samples containing increasingly different levels of newly synthesized Omp F protein were subjected to chemical cross-linking with a bifunctional cleavable reagent in order to investigate the process of trimer formation of the protein. From the results obtained, we conclude that the formation of functional Omp F trimers is substantially delayed to, and can be distinguished from, the incorporation of Omp F monomers to the outer membrane.