Direct participation of lexA protein in repression of colicin E1 synthesis

SOS-Independent Pyocin Production in P. aeruginosa Is Induced by XerC Recombinase Deficiency

Pyocins are phage tail-like protein complexes that can be used by Pseudomonas aeruginosa to enact intraspecies competition by killing competing strains. The pyocin gene cluster also encodes holin and lysin enzymes that lyse producer cells to release the pyocins. The best-known inducers of pyocin production under laboratory conditions are DNA-damaging agents, including fluoroquinolone antibiotics, that activate the SOS response. Here, we report the discovery of an alternate, RecA-independent pathway of strong pyocin induction that is active in cells deficient for the tyrosine recombinase XerC. When ΔxerC cells were examined at the single-cell level, only a fraction of the cell population strongly expressed pyocins before explosively lysing, suggesting a that a built-in heterogenous response system protects the cell population from widespread lysis. Disabling the holin and lysin enzymes or deleting the entire pyocin gene cluster blocked explosive lysis and delayed but did not prevent the death of pyocin-producing cells, suggesting that ΔxerC cells activate other lysis pathways. Mutating XerC to abolish its recombinase activity induced pyocin expression to a lesser extent than the full deletion, suggesting that XerC has multiple functions with respect to pyocin activation. Our studies uncover a new pathway for pyocin production and highlight its response across a genetically identical population. Moreover, our finding that ΔxerC populations are hypersensitive to fluoroquinolones raises the intriguing possibility that XerC inhibition may potentiate the activity of these antibiotics against P. aeruginosa infections. IMPORTANCE Pseudomonas aeruginosa is a versatile and ubiquitous bacterium that frequently infects humans as an opportunistic pathogen. P. aeruginosa competes with other strains within the species by producing killing complexes termed pyocins, which are only known to be induced by cells experiencing DNA damage and the subsequent SOS response. Here, we discovered that strains lacking a recombinase enzyme called XerC strongly produce pyocins independently of the SOS response. We also show that these strains are hypersensitive to commonly used fluoroquinolone antibiotic treatment and that fluoroquinolones further stimulate pyocin production. Thus, XerC is an attractive target for future therapies that simultaneously sensitize P. aeruginosa to antibiotics and stimulate the production of bactericidal pyocins.

The Use and Abuse of LexA by Mobile Genetic Elements

The SOS response is an essential process for responding to DNA damage in bacteria. The expression of SOS genes is under the control of LexA, a global transcription factor that undergoes self-cleavage during stress to allow the expression of DNA repair functions and delay cell division until the damage is rectified. LexA also regulates genes that are not part of this cell rescue program, and the induction of bacteriophages, the movement of pathogenicity islands, and the expression of virulence factors and bacteriocins are all controlled by this important transcription factor. Recently it has emerged that when regulating the expression of genes from mobile genetic elements (MGEs), LexA often does so in concert with a corepressor. This accessory regulator can either be a host-encoded global transcription factor, which responds to various metabolic changes, or a factor that is encoded for by the MGE itself. Thus, the coupling of LexA-mediated regulation to a secondary transcription factor not only detaches LexA from its primary SOS role, but also fine-tunes gene expression from the MGE, enabling it to respond to multiple stresses. Here we discuss the mechanisms of such coordinated regulation and its implications for cells carrying such MGEs.

Double locking of an Escherichia coli promoter by two repressors prevents premature colicin expression and cell lysis

The synthesis of Eschericha coli colicins is lethal to the producing cell and is repressed during normal growth by the LexA transcription factor, which is the master repressor of the SOS system for repair of DNA damage. Following DNA damage, LexA is inactivated and SOS repair genes are induced immediately, but colicin production is delayed and induced only in terminally damaged cells. The cause of this delay is unknown. Here we identify the global transcription repressor, IscR, as being directly responsible for the delay in colicin K expression during the SOS response, and identify the DNA target for IscR at the colicin K operon promoter. Our results suggest that, IscR stabilizes LexA at the cka promoter after DNA damage thus, preventing its cleavage and inactivation, and this cooperation ensures that suicidal colicin K production is switched on only as a last resort. A similar mechanism operates at the regulatory region of other colicins and, hence, we suggest that many promoters that control the expression of 'lethal' genes are double locked.

Colicin biology

Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed.

Increased production of colicin E1 in stationary phase

The synthesis of colicin E1 is known to be regulated by the SOS response, anaerobiosis, and catabolite repression. The expression of cea-lacZ fusions was also found to be stimulated when cells reached stationary phase. This increase in expression was determined to be due to depletion of nutrients from the medium, since the addition of fresh medium reversed the effect. Expression of the fusion increased when cells were starved in 10 mM MgSO4 and when they were grown in conditioned medium in which cells had been grown previously. The stimulation of expression occurred when the cea-lacZ fusion was present in single-copy as well as in multicopy plasmids. Finally, the data were consistent with this increase being independent of the SOS response, anaerobiosis, catabolite repression, and integration host factor as well as the stationary-phase regulators encoded by rpoS and lrp.

Regulation of pyocin genes in Pseudomonas aeruginosa by positive (prtN) and negative (prtR) regulatory genes

Most strains of Pseudomonas aeruginosa produce various types of bacteriocins (pyocins), namely, R-, F-, and S-type pyocins. The production of all types of pyocins was shown to be regulated by positive (prtN) and negative (prtR) regulatory genes. The prtN gene activates the expression of various pyocin genes, probably by the interaction of its product with the DNA sequences conserved in the 5' noncoding regions of the pyocin genes. The prtR gene represses the expression of the prtN gene, and its product, predicted from the nucleotide sequence, has a structure characteristic of phage repressors and seems to be inactivated by the RecA protein activated by DNA damage. A model for the regulation of the pyocin genes is proposed.

Anaerobic control of colicin E1 production

Expression of the cea gene, which is carried by the ColE1 plasmid and which encodes colicin E1, was found to be greatly increased when the cells were grown anaerobically. By using cea-lacZ fusions to quantitate expression, aerobic levels were found to be only a few percent of the anaerobic levels. The anaerobic increase in expression was observed both in protein and in operon fusions, indicating that its regulation occurred at the level of transcription. It was also found to require a functional fnr gene and to occur when the cea-lacZ fusion was present as a single copy in the bacterial chromosome instead of in the multicopy ColE1 plasmid. Anaerobic expression was regulated by the SOS response and catabolite repression as is aerobic expression. The start site of the mRNA produced under anaerobic conditions was mapped by primer extension and found to be the same as the start for mRNA produced under aerobic conditions. These observations show that the cea gene is anaerobically regulated and that the Fnr protein is a positive regulator of transcription of this gene.

Lactococcin A, a new bacteriocin from Lactococcus lactis subsp. cremoris: isolation and characterization of the protein and its gene

A new bacteriocin, termed lactococcin A (LCN-A), from Lactococcus lactis subsp. cremoris LMG 2130 was purified and sequenced. The polypeptide contained no unusual amino acids and showed no significant sequence similarity to other known proteins. Only lactococci were killed by the bacteriocin. Of more than 120 L. lactis strains tested, only 1 was found resistant to LCN-A. The most sensitive strain tested, L. lactis subsp. cremoris NCDO 1198, was inhibited by 7 pM LCN-A. By use of a synthetic DNA probe, lcnA was found to be located on a 55-kb plasmid. The lcnA gene was cloned and sequenced. The sequence data revealed that LCN-A is ribosomally synthesized as a 75-amino-acid precursor including a 21-amino-acid N-terminal extension. An open reading frame encoding a 98-amino-acid polypeptide was found downstream of and in the same operon as lcnA. We propose that this open reading frame encodes an immunity function for LCN-A. In Escherichia coli lcnA did not cause an LCN-A+ phenotype. L. lactis subsp. lactis IL 1403 produced small amounts of the bacteriocin and became resistant to LCN-A after transformation with a recombinant plasmid carrying lcnA. The other lactococcal strains transformed with the same recombinant plasmid became resistant to LCN-A but did not produce any detectable amount of the bacteriocin.

Induction of inhibitory agent produced by Enterococcus faecalis

The effect of treatment with inducing agents, such as mitomycin C, hydrogen peroxide and UV irradiation on the production of two inhibitors by different mutants from Enterococcus faecalis S-48 was studied. With hydrogen peroxide and UV light no increase in either the absolute or the relative amount of antagonistic substances was observed. With mitomycin C, a significant increase in the individual cell capacity for inhibitor production was detected.

Increase in plasmid transformation efficiency in SOS-induced Escherichia coli cells

UV irradiation of competent cells of Escherichia coli K12 produced an increase in the efficiency of transformation with plasmid DNA. This phenomenon has been called IPTE (increase in plasmid transformation efficiency) and is dependent on the activated state of the RecA protein. IPTE is independent of the lexA, recB recC, and recF genes. It is not related to the size or the replicon type of the plasmid. Furthermore, it is also induced in cells which have been previously treated with other SOS system-inducing agents such as bleomycin, mitomycin C, or nalidixic acid. IPTE is therefore similar to other repair (SOS) functions inducible by DNA damage since all of them are dependent upon activation of the RecA protein. IPTE differs from other SOS functions in the absence of a direct control by the LexA repressor.

Temporal control of colicin E1 induction

The expression of the gene encoding colicin E1, cea, was studied in Escherichia coli by using cea-lacZ gene fusions. Expression of the fusions showed the same characteristics as those of the wild-type cea gene: induction by treatments that damage DNA and regulation by the SOS response, sensitivity to catabolite repression, and a low basal level of expression, despite the presence of the fusion in a multicopy plasmid. Induction of expression by DNA-damaging treatments was found to differ from other genes involved in the SOS response (exemplified by recA), in that higher levels of DNA damage were required and expression occurred only after a pronounced delay. The delay in expression following an inducing treatment was more pronounced under conditions of catabolite repression, indicating that the cyclic AMP-cyclic AMP receptor protein complex may play a role in induction. These observations also suggest a biological rationale for the control of cea expression by the SOS response and the cyclic AMP-cyclic AMP receptor protein catabolite repression system.

Synthesis of linear plasmid multimers in Escherichia coli K-12

Linear plasmid multimers were identified in extracts of recB21 recC22 strains containing derivatives of the ColE1-type plasmids pACYC184 and pBR322. A mutation in sbcB increases the proportion of plasmid DNA as linear multimers. A model to explain this is based on proposed roles of RecBC enzyme and SbcB enzyme (DNA exonuclease I) in preventing two types of rolling-circle DNA synthesis. Support for this hypothesis was obtained by derepressing synthesis of an inhibitor of RecBC enzyme and observing a difference in control of linear multimer synthesis and monomer circle replication. Reinitiation of rolling-circle DNA synthesis was proposed to occur by recA+-dependent and recA+-independent recombination events involving linear multimers. The presence of linear plasmid multimers in recB and recC mutants sheds new light on plasmid recombination frequencies in various mutant strains.

Expression of the Plasmodium knowlesi circumsporozoite antigen in Escherichia coli directed by Plasmodium bacterial-like promoter sequences

The Plasmodium knowlesi circumsporozoite (CS) gene is expressed in Escherichia coli directly from a parasite genomic DNA fragment, using promoter and ribosome-binding site (RBS) sequences present in this fragment. Transcription of the CS gene in E. coli is directed by tandem Plasmodium bacterial-like promoter elements located within the 0.5-kb EcoRI-HindIII fragment roughly 2.5 kb 5' from the CS gene within the 11-kb EcoRI parasite genomic DNA fragment. No readthrough from vector promoters or fortuitous promotion from plasmodial A + T-rich sequences was observed. The endogenous Plasmodium promoter of the CS gene does not seem to be recognized by E. coli RNA polymerases. Two tandem E. coli-recognized promoters are relatively strong judging by their ability to drive the bacterial chloramphenicol acetyl-transferase (CAT) gene. Translation of the message must be achieved by utilising an AAGAA sequence 4 bp 5' from the ATG initiation codon as RBS.

Structural and functional organization of the colicin E1 operon

We analyzed the structural and functional relationships among independently cloned segments of the plasmid ColE1 region that regulates and codes for colicin E1 (cea), immunity (imm), and the mitomycin C-induced lethality function (lys). On the basis of physiological properties, restriction endonuclease mapping, and DNA sequence analysis, the following recombinant plasmids were determined to represent three major functional classes: pNP12 (cea+, imm+, lys+), pNP4 (cea+, imm+, lys-), and pNP6 (cea+, imm-, lys-). Our results have established the order, boundaries, and relative orientation of the three structural genes, the location of the promoter region for imm gene transcription, and the predicted amino acid sequences of the imm and lys gene products. Hydropathicity analysis suggests that both proteins have hydrophobic amino acid segments characteristic of membrane-associated proteins. A model for the structure and expression of the colicin E1 operon is proposed in which the cea and lys genes are expressed from a single inducible promoter that is controlled by the lexA repressor in response to the SOS system of Escherichia coli. The imm gene lies between the cea and lys genes and is expressed by transcription in the opposite direction from a promoter located within the lys gene. This arrangement of structural genes indicates that the transcriptional units for all three genes overlap. We suggest that the formation of anti-sense RNA may be an important element in the coordinate regulation of gene expression in this system.

Spontaneous induction of colicin E1 in Escherichia coli strains deficient in both exonucleases I and V

Colicin E1 synthesis is spontaneously induced in pRSF2124-carrying strains of Escherichia coli deficient in exonucleases I (sbcB) and V (recB recC). In contrast, the specific activity of beta-lactamase, which is also encoded by pRSF2124, is not affected by the absence of these enzymes. These results suggest that colicin E1 induction is specific and does not result either from a significant change in overall plasmid transcription or copy number. Furthermore, the level of spontaneous induction was similar to that obtained with mitomycin C.

recA is required in the induction of pectin lyase and carotovoricin in Erwinia carotovora subsp. carotovora

Pectin lyase (PNL) and the bacteriocin carotovoricin (CTV) were induced in Erwinia carotovora subsp. carotovora 71 by the DNA-damaging agents mitomycin C, nalidixic acid, and UV light. To determine whether the recA product was involved in the expression of these damage-inducible phenotypes, we cloned the E. carotovora subsp. carotovora recA+ gene, inactivated it by Tn5 insertion, and constructed an E. carotovora subsp. carotovora recA::Tn5 strain by gene replacement via homologous recombination. The RecA- strain was more sensitive to methyl methanesulfonate, nitroquinoline oxide, and UV light than its RecA+ parent. The recA mutation did not affect the production of pectate lyase, polygalacturonase, cellulase, and protease or the ability to cause soft rot of potato tubers. With this mutant, unlike with the RecA+ parent strain, PNL and CTV were not induced by mitomycin C or detected in potato tuber tissue. The RecA+ phenotype, including the inducibility of PNL and CTV, could, however, be restored in the mutant in trans by the recA+ gene from either E. carotovora subsp. carotovora or Escherichia coli. We conclude that, in E. carotovora subsp. carotovora, the recA product is required in the induction of PNL and CTV.

Positive regulation of the colicin E1 gene by cyclic AMP and cyclic AMP receptor protein

In previous experiments, we showed that the in vivo transcription of the colicin E1 gene was dependent on cyclic AMP in adenylate cyclase-defective mutant cells of Escherichia coli (Ebina, Y. and Nakazawa, A (1983) J. Biol. Chem. 258, 7072-7078). We now show that cyclic AMP and cyclic AMP receptor protein stimulated the in vitro transcription of the gene in the presence of spermidine. As determined in DNase I protection experiments, two binding sites for the complex of cyclic AMP and the receptor protein were identified about 60 base pairs (CRP-1) and 110 base pairs (CRP-2) upstream from the transcription initiation site of the colicin E1 gene. CRP-1 had a higher affinity for the complex than that of CRP-2. Substituting an unrelated DNA sequence for CRP-2 reduced the efficiency of in vitro stimulation of the gene by cyclic AMP and the receptor protein. These potential binding sites for the cyclic AMP-cyclic AMP receptor protein complex probably participate in the stimulation of the colicin E1 gene transcription.

Colicin synthesis and cell death

Colicin E1 is a small plasmid, containing the cea gene for colicin, the most prominent product of the plasmid. Colicin is a 56-kilodalton bacteriocin which is especially toxic to Escherichia coli cells that do not contain the plasmid. Under normal growth conditions very low levels of the plasmid are produced as a result of cea gene repression by the host LexA protein. Conditions that lower the concentration of LexA protein result in elevated levels of colicin synthesis. The LexA protein concentration can be lowered by exposing the cells to DNA-damaging reagents such as UV light or mitomycin C. This is because DNA damage signals the host SOS response; the response leads to activation of the RecA protease which degrades the LexA protein. DNA-damaging reagents result in very high levels of colicin synthesis and subsequent death of plasmid-bearing cells. Elevated levels of colicin are also produced in mutants of E. coli that are deficient in LexA protein. We found that comparably high levels of colicin can be produced in such mutants in the absence of cell death. In lexA strains carrying a defective LexA repressor, colicin synthesis shows a strong temperature dependence. Ten to twenty times more colicin is synthesized at 42 degrees C. This sharp dependence of synthesis on temperature suggests that there are factors other than the LexA protein which regulate colicin synthesis.

Colicin E3 and its immunity genes

A DNA segment of plasmid ColE3-CA38 was cloned into pBR328 and its nucleotide sequence was determined. This segment contains the putative promoter-operator region, the structural genes of protein A (gene A) and protein B (gene B) of colicin E3, and a part of gene H. Just behind the promoter region, there is an inverted repeat structure of two 'SOS boxes', the specific binding site of the lexA protein. This suggests that the expression of colicin E3 is regulated directly by the lexA protein. Genes A and B face the same direction, with an intergenic space of nine nucleotides between them. ColE3-CA38 and ColE1-K30 are homologous in their promoter-operator regions, but hardly any homology was found in their structural genes. On the other hand, ColE3-CA38 is fairly homologous to CloDF13 throughout the regions sequenced, with some exceptions including putative receptor-binding regions. By deletion mapping of the immunity gene and recloning of gene B, it was shown genetically that protein B itself is the actual immunity substance of colicin E3. It was also found that the expression of E3 immunity partially depends on the recA function. Thus, we propose two modes of expression of E3 immunity: in the uninduced state, only a slight amount of protein B is produced constitutively to protect the cell from being attacked by the exogenous colicin; and in the SOS-induced state, a large amount of protein B is produced to protect the protein synthesis system of the host cell from ribosome inactivation by endogenously produced colicin E3.

Structure and expression of the ColE2-P9 immunity gene

The primary structure and expression of the ColE2-P9 immunity gene (imm) were investigated. The imm gene is located behind the colicin gene (col) in the same orientation with an intergenic space of two base pairs. Although the imm gene was transcribed primarily in response to the SOS function of the host cell as well as the col gene, the immunity phenotype also appeared to be expressed by only a slight level of leaky transcription without an evident promoter. On comparing the ColE2-P9 sequence with those of relevant plasmids, a highly homologous sequence with the immE2 gene was found downstream of the immE3 gene of ColE3-CA38, and thus, an evolutional relationships could be deduced among some E-group Col plasmids.

Analysis of a cloned colicin Ib gene: complete nucleotide sequence and implications for regulation of expression

The complete nucleotide sequence of a 2,971 base pair EcoRI fragment carrying the structural gene for colicin Ib has been determined. The length of the gene is 1,881 nucleotides which is predicted to produce a protein of 626 amino acids and of molecular weight 71,364. The structural gene is flanked by likely promoter and terminator signals and in between the promoter and the ribosome binding site is an inverted repeat sequence which resembles other sequences known to bind the LexA protein. Further analysis of the 5' flanking sequences revealed a second region which may act either as a second LexA binding site and/or in the binding of cyclic AMP receptor protein. Comparison of the predicted amino acid sequence of colicin Ib with that of colicins A and E1 reveals localised homology. The implications of these similarities in the proteins and of regulation of the colicin Ib structural gene are discussed.

Genetic analysis of the functional relationship between colicin E3 and its immunity protein

Partial deletions in the immunity gene of the colicin E3 operon were used to study possible functions of the immunity protein besides protection against exogenous colicin. Nuclease BAL-31 was used to create a series of carboxyl-terminal deletions of the immunity gene. Mutants displaying lowered immunity against exogenous colicin were found, and six that had reduced but detectable levels of immunity were chosen for further analysis. DNA sequence analysis of the deletions showed that all six terminated within the last five codons of the immunity gene. The wild-type immunity gene was replaced by each of the six mutated immunity genes in a plasmid containing an otherwise functional colicin E3 operon. Transformants containing the resulting plasmids produced smaller colonies on solid medium and grew more slowly in liquid culture than transformants carrying the wild-type colicin and immunity genes. This result suggested that immunity protein was required to protect the cell against endogenous colicin E3. This idea was confirmed in experiments in which the colicin E3 and immunity genes were independently cloned on two compatible plasmid vectors.

Promotion, termination, and anti-termination in the rpsU-dnaG-rpoD macromolecular synthesis operon of E. coli K-12

The regulatory regions for the rpsU-dnaG-rpoD macromolecular synthesis operon have been fused to a structural gene whose product is readily assayed (the Cmr structural gene coding for chloramphenicol acetyl transferase, CAT). The promoters (P1, P2, P3, Pa, Pb, Phs) for the macromolecular synthesis operon have different strengths as shown by their relative abilities to drive expression of the CAT gene. Promoter occlusion by P1 can be demonstrated within this operon. Regions 5kb upstream have a profound effect on operon gene expression. There is a thermoinducible promoter located within the dnaG structural gene. One of the macromolecular synthesis operon promoters is under lexA control. Although the operon structure allows coordinate expression of rpsU, dnaG and rpoD these additional features suggest that expression of individual genes can be independently regulated in response to altered growth conditions.

The use of transposon Tn5 mutagenesis in the rapid generation of correlated physical and genetic maps of DNA segments cloned into multicopy plasmids--a review

The properties of transposon Tn5 that render it useful for in vivo mutagenesis of cloned DNA sequences are reviewed. Transposition frequency, insertional specificity, polarity and stability of Tn5 insertion mutations are among the topics discussed. Examples are cited from the published literature which illustrate the applications of Tn5 mutagenesis to the analysis of cloned prokaryotic and eukaryotic genes. A methods section is included which outlines precisely how to carry out transposon Tn5 mutagenesis analysis of cloned DNA segments.

Plasmid ColE3 specifies a lysis protein

Tn5 insertion mutations in plasmid ColE3 were isolated and characterized. Several of the mutants synthesized normal amounts of active colicin E3 but, unlike wild-type colicinogenic cells, did not release measurable amounts of colicin into the culture medium. Cells bearing the mutant plasmids were immune to exogenous colicin E3 at about the same level as wild-type colicinogenic cells. All of these lysis mutants mapped near, but outside of, the structural genes for colicin E3 and immunity protein. Cells carrying the insertion mutations which did not release colicin E3 into the medium were not killed by UV exposure at levels that killed cells bearing wild-type plasmids. The protein specified by the lysis gene was identified in minicells and in mitomycin C-induced cells. A small protein, with a molecular weight between 6,000 and 7,000, was found in cells which released colicin into the medium, but not in mutant cells that did not release colicin. Two mutants with insertions within the structural gene for colicin E3 were also characterized. They produced no colicin activity, but both synthesized a peptide consistent with their map position near the middle of the colicin gene. These two insertion mutants were also phenotypically lysis mutants--they were not killed by UV doses lethal to wild-type colicinogenic cells and they did not synthesize the small putative lysis protein. Therefore, the lysis gene is probably in the same operon as the structural gene for colicin E3.

UV inducible UV protection and mutation functions on the I group plasmid TP110

The UV protection and mutation properties of the I group plasmid TP110 have been investigated. It is demonstrated that the genes responsible for these effects are able to complement the deficiency in umuC36 mutants of E. coli, as are the similar genes carried by the B group plasmid R16. Mu-lac inserts into TP110 have been isolated which abolish the UV protection and mutation functions. Restriction mapping of these inserts locates them within a single region of the genome. A comparison of the restriction sites of this region with the muc region of pKM101 reveals very little similarity. Expression of beta-galactosidase in those Mu-lac inserts in which the lacZ gene is fused to the promoter for the protection and mutation functions is inducible by DNA damaging agents, and induction in mutant strains suggests that these genes are under the direct control of the lexA repressor.

Plasmid-encoded regulation of colicin E1 gene expression

A plasmid-encoded factor that regulates the expression of the colicin E1 gene was found in molecular cloning experiments. The 2,294-base-pair AvaII fragment of the colicin E1 plasmid (ColE1) carrying the colicin E1 structural gene and the promoter-operator region had the same information with respect to the repressibility and inducibility of colicin E1 synthesis as the original ColE1 plasmid. An operon fusion was constructed between the 204-bp fragment containing the colicin E1 promoter-operator and xylE, the structural gene for catechol 2,3-dioxygenase encoded on the TOL plasmid of Pseudomonas putida. The synthesis of the dioxygenase from the resulting plasmid occurred in recA+, but not in recA- cells and was derepressed in the recA lexA(Def) double mutant. These results indicate that the ColE1 plasmid has no repressor gene for colicin E1 synthesis and that the lexA protein functions as a repressor. Colicin E1 gene expression was adenosine 3',5'-phosphate (cAMP) dependent. Upon the removal of two PvuII fragments (2,000 bp in length) from the ColE1 plasmid, the induced synthesis of colicin E1 occurred in the adenylate-cyclase mutant even without cAMP. The 3,100-bp Tth111I fragment of the ColE1 plasmid cloned on pACYC177 restored the cAMP dependency of the deleted ColE1 plasmid. Since the deleted fragments correspond to the mobility region of ColE1, the cAMP dependency of the gene expression should be somehow related to the plasmid mobilization function.

Complete nucleotide sequence of the structural gene for colicin A, a gene translated at non-uniform rate

The complete nucleotide sequence of the structural gene for colicin A has been established. This sequence consists of 1776 base-pairs. According to the predicted amino acid sequence, the colicin A polypeptide chain comprises 592 amino acids and has a molecular weight of 62,989. The amino-terminal part is rich in proline and glycine and accordingly secondary structure prediction indicates that this region (1 to 185) is beta-structured. The rest of the molecule (residues 186 to 592) is very rich in alpha-helix. An uncharged amino acid sequence of 48 residues is located in the C-terminal part of the molecule, which is involved in the membrane depolarization caused by colicin A. A similar region has been found in colicin E1, which has the same mode of action as colicin A. Three peptides of these bacteriocins were found to be homologous, but a comparison of the bacteriocin genes did not reveal any significant homology out of the corresponding regions. The codon usage of both genes, however, exhibits some similarity and is quite different from that of genes coding for highly or weakly expressed proteins of Escherichia coli.

Structure of the plasmodium knowlesi gene coding for the circumsporozoite protein

The gene that codes for the surface antigen of Plasmodium knowlesi sporozoites (CS protein) is unsplit and present in the genome in only one copy. The CS protein, as deduced from DNA sequence analysis of the structural gene, has an unusual structure with the central 40% of the polypeptide chain present as 12 tandemly repeated amino acid peptide units flanked by regions of highly charged amino acids. The protein has an amino-terminal hydrophobic amino acid signal sequence and a hydrophobic carboxy-terminal anchor sequence. The coding sequence of the gene has an AT content of 53%, compared with 70% AT in the 5' and 3' flanking sequences, and is contained entirely within an 11 kb Eco RI genomic DNA fragment. This genomic fragment expresses the CS protein in E. coli, indicating that the parasite promoter and ribosome binding site signals can be recognized in E. coli.

The SOS system of Escherichia coli in the regulation of bacteriophage lambda development

A sequence homologous to the known SOS boxes is found in the Po promoter of phage lambda. It is suggested that the sequence found is a binding site for the LexA repressor. The mechanism of the LexA part in regulation of lambda development is discussed. It is based on the competitive transcription of the RNA encoding CII protein and the short OOP-RNA transcribed from the Po promoter.

Regulation of expression of the colicin gene of I1 group plasmid TP110

The control of expression of the colicin Ib gene of the I1 group plasmid TP110 has been investigated. The colicin promoter was fused to the structural gene for beta-galactosidase, using the Mu d(Aprlac) phage, and the plasmid carrying this fusion was introduced into a variety of bacterial strains defective in genes involved in the "SOS" response. Colicin Ib belongs to that group of genes directly controlled by the repressor produced by the lexA gene, and expression was inducible by DNA-damaging agents. Mutations in uvrA, -B, and -C reduced the efficiency of induction by mitomycin C, as did mutations in recB. Mutations in recA and recF effectively prevented induction by mitomycin C, whereas mutations in lexA had contrasting effects, depending upon their effect on the properties of lexA protein. The spr-51 mutation (which inactivates lexA protein) led to constitutive expression, whereas the lexA3 mutation (which makes lexA protein refractory to cleavage by recA protein) completely inhibited inducible expression. In addition to lexA control, a TP110-coded function was identified which appeared able to inhibit colicin expression when the gene responsible was present in high copy number.

Autoinduced synthesis of colicin E2

Escherichia coli K-12 cells carrying the high copy number plasmid ColE2-P9 and a sfiA-lacZ gene fusion exhibit abnormally high levels of SOS-regulated phi sfiA-lacZ expression. Increased sfiA-lacZ expression is caused by the action of colicin E2, which is a DNase, rather than by the presence of multiple copies of a binding site for LexA protein, the repressor for the sfiA and colicin E2 genes. Expression of sfiA-lacZ was reduced to normal levels if the ColE2+ strain lacked the outer membrane colicin E2 receptor protein (BtuB) or if they carried an increased number of colicin E2 immunity genes. The results suggest that cultures of ColE2+ strains contain a small number of cells which produce colicin which can then enter other, non-producing cells in the culture and cause sufficient damage to the DNA to induce the SOS system. The levels of colicin E2 immunity in the producing cells is presumably sufficient to prevent extensive lethal effects of the colicin, but insufficient to prevent limited endonuclease activity. An important consequence of this phenomenon is that the DNase action of colicin E2 can stimulate its own production.

Molecular structure and function of the bacteriocin gene and bacteriocin protein of plasmid Clo DF13

In this paper we present the complete nucleotide sequence of the bacteriocin gene of plasmid Clo DF13. According to the predicted aminoacid sequence the bacteriocin, cloacin DF13, consists of 561 aminoacids and has a molecular weight of 59,293 D. To obtain insight into the structure and function of specific parts of the cloacin molecule, we constructed a hydration profile and we predicted the secondary structure of the protein. According to our predictions, the N-terminus of cloacin DF13 (corresponding to the first 150-180 aminoacids) is relatively hydrophobic and is rich in glycine residues. The data obtained support previous findings that the N-terminal part of cloacin DF13 is involved in translocation of this protein across the cell membrane. The C-terminal part of the cloacin protein is rich in positively charged aminoacids; this might reflect the RNase activity located within this domain. A comparison of the bacteriocin genes and corresponding proteins of Clo DF13 and Col E1 did not reveal any homology at the level of either the nucleotide or the aminoacid sequence. The codon usage of both genes, however, exhibits striking similarities. The sequence data obtained during this study enabled us to present the nucleotide sequence of the entire cloacin operon. The structure of this operon and the regulation of expression of the genes, located within this operon, is discussed.

The coupled use of 'footprinting' and exonuclease III methodology for RNA polymerase binding and initiation. Application for the analysis of three tandem promoters at the control region of colicin El

In order to determine the initiation site for three promoters P1, P2 and P3 (5' to 3') in close proximity in the colicin E1 control region we developed a new methodology that couples ternary complex formation and the analysis of the 3' border protected from exonuclease III digestion. The initiation of transcription could be detected by measuring the shift in the position of the 3' protected border when RNA polymerase moved from its binary complex position to its ternary complex position. The latter stops at a specific nucleotide because transcription is initiated with one or more NTPs missing. This approach, coupled with "footprinting", can also be used to decide whether the formation of an RNA polymerase binary or ternary complex at one site excludes or weakens binding at neighboring sites. The location of 3' protected borders reveals the formation of respective binary and ternary complexes at non-saturating RNA polymerase conditions, whereas at saturating conditions only the distal 3' boundary is seen and exonuclease cannot penetrate further. However, if "footprinting" reveals proximal 5' patterns this establishes that simultaneous binding has occurred on the same DNA fragment. The data showed that this was true for P1 and P3 which are only 8 nucleotides apart. P2 could only be detected at non-saturating conditions since it overlaps both P1 and P3. The evidence from the literature and this study establishes P1 as the true colicin E1 promoter with the possibility that supercoiling may eliminate any role for P2 and P3.