Copy-number mutants of the plasmid carrying the replication origin of the Escherichia coli chromosome: evidence for a control region of replication

Toxin-Antidote Elements Across the Tree of Life

In life's constant battle for survival, it takes one to kill but two to conquer. Toxin-antitoxin or toxin-antidote (TA) elements are genetic dyads that cheat the laws of inheritance to guarantee their transmission to the next generation. This seemingly simple genetic arrangement—a toxin linked to its antidote—is capable of quickly spreading and persisting in natural populations. TA elements were first discovered in bacterial plasmids in the 1980s and have recently been characterized in fungi, plants, and animals, where they underlie genetic incompatibilities and sterility in crosses between wild isolates. In this review, we provide a unified view of TA elements in both prokaryotic and eukaryotic organisms and highlight their similarities and differences at the evolutionary, genetic, and molecular levels. Finally, we propose several scenarios that could explain the paradox of the evolutionary origin of TA elements and argue that these elements may be key evolutionary players and that the full scope of their roles is only beginning to be uncovered.

Identification of a Formate-Dependent Uric Acid Degradation Pathway in Escherichia coli

Purine is a nitrogen-containing compound that is abundant in nature. In organisms that utilize purine as a nitrogen source, purine is converted to uric acid, which is then converted to allantoin. Allantoin is then converted to ammonia. In Escherichia coli, neither urate-degrading activity nor a gene encoding an enzyme homologous to the known urate-degrading enzymes had previously been found. Here, we demonstrate urate-degrading activity in E. coli We first identified aegA as an E. coli gene involved in oxidative stress tolerance. An examination of gene expression revealed that both aegA and its paralog ygfT are expressed under both microaerobic and anaerobic conditions. The ygfT gene is localized within a chromosomal gene cluster presumably involved in purine catabolism. Accordingly, the expression of ygfT increased in the presence of exogenous uric acid, suggesting that ygfT is involved in urate degradation. Examination of the change of uric acid levels in the growth medium with time revealed urate-degrading activity under microaerobic and anaerobic conditions in the wild-type strain but not in the aegA ygfT double-deletion mutant. Furthermore, AegA- and YgfT-dependent urate-degrading activity was detected only in the presence of formate and formate dehydrogenase H. Collectively, these observations indicate the presence of urate-degrading activity in E. coli that is operational under microaerobic and anaerobic conditions. The activity requires formate, formate dehydrogenase H, and either aegA or ygfT We also identified other putative genes which are involved not only in formate-dependent but also in formate-independent urate degradation and may function in the regulation or cofactor synthesis in purine catabolism.IMPORTANCE The metabolic pathway of uric acid degradation to date has been elucidated only in aerobic environments and is not understood in anaerobic and microaerobic environments. In the current study, we showed that Escherichia coli, a facultative anaerobic organism, uses uric acid as a sole source of nitrogen under anaerobic and microaerobic conditions. We also showed that formate, formate dehydrogenase H, and either AegA or YgfT are involved in uric acid degradation. We propose that formate may act as an electron donor for a uric acid-degrading enzyme in this bacterium.

Purification and crystallization of Vibrio fischeri CcdB and its complexes with fragments of gyrase and CcdA

The ccd toxin-antitoxin module from the Escherichia coli F plasmid has a homologue on the Vibrio fischeri integron. The homologue of the toxin (CcdB(Vfi)) was crystallized in two different crystal forms. The first form belongs to space group I23 or I2(1)3, with unit-cell parameter a = 84.5 A, and diffracts to 1.5 A resolution. The second crystal form belongs to space group C2, with unit-cell parameters a = 58.5, b = 43.6, c = 37.5 A, beta = 110.0 degrees, and diffracts to 1.7 A resolution. The complex of CcdB(Vfi) with the GyrA14(Vfi) fragment of V. fischeri gyrase crystallizes in space group P2(1)2(1)2(1), with unit-cell parameters a = 53.5, b = 94.6, c = 58.1 A, and diffracts to 2.2 A resolution. The corresponding mixed complex with E. coli GyrA14(Ec) crystallizes in space group C2, with unit-cell parameters a = 130.1, b = 90.8, c = 58.1 A, beta = 102.6 degrees, and diffracts to 1.95 A. Finally, a complex between CcdB(Vfi) and part of the F-plasmid antitoxin CcdA(F) crystallizes in space group P2(1)2(1)2(1), with unit-cell parameters a = 46.9, b = 62.6, c = 82.0 A, and diffracts to 1.9 A resolution.

The bacterial ParA-ParB partitioning proteins

A pair of genes designated parA and parB are encoded by many low copy number plasmids and bacterial chromosomes. They work with one or more cis-acting sites termed centromere-like sequences to ensure better than random predivisional partitioning of the DNA molecule that encodes them. The centromere-like sequences nucleate binding of ParB and titrate sufficient protein to create foci, which are easily visible by immuno-fluorescence microscopy. These foci normally follow the plasmid or the chromosomal replication oriC complexes. ParA is a membrane-associated ATPase that is essential for this symmetric movement of the ParB foci. In Bacillus subtilis ParA oscillates from end to end of the cell as does MinD of E. coli, a relative of the ParA family. ParA may facilitate ParB movement along the inner surface of the cytoplasmic membrane to encounter and become tethered to the next replication zone. The ATP-bound form of ParA appears to adopt the conformation needed to drive partition. Hydrolysis to create ParA-ADP or free ParA appears to favour a form that is not located at the pole and binds to DNA rather than the partition complex. Definition of the protein domains needed for interaction with membranes and the conformational changes that occur on interaction with ATP/ADP will provide insights into the partitioning mechanism and possible targets for inhibitors of partitioning.

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.

Membrane association of active plasmid partitioning protein A in Escherichia coli

QsopA and SopA, proteins essential for stable maintenance of low copy number plasmids and encoded on plasmid QpH1 of Coxiella burnetii and the F plasmid of Escherichia coli, respectively, are shown to be membrane associated using three independent approaches: isolation of hybrid protein A-PhoA proteins that display PhoA (bacterial alkaline phosphatase) activity indicating a periplasmic location, biochemical fractionation by flotation gradient centrifugation, and subcellular localization by immunoelectron microscopy. These data provide insight into the mechanism by which partitioning protein A spatially directs plasmids into daughter cells at bacterial division.

Identification of a partition region carried by the plasmid QpH1 of Coxiella burnetii

Coxiella burnetii is an intracellular bacterial pathogen which causes Q fever in humans and other animals. Most of the isolates found carry plasmids which share considerable homology. Unfortunately all of these plasmids remain cryptic. Initial attempts to look for secreted or membrane proteins encoded by these plasmids using TnphoA mutagenesis revealed an open reading frame on the EcoRI-fragment C of the plasmid QpH1. Upstream DNA sequencing of the TnphoA insertions revealed a deduced peptide sequence with homology to the SopA protein which is encoded by the F plasmid in Escherichia coli. Maxicell analysis showed that fragment C encoded two proteins: one was 43.5 kDa in size and designated QsopA, and a second was 38 kDa in size. These proteins are similar in molecular weight to the SopA and SopB proteins, which are essential components of the partition mechanism of the F plasmid. The region appears to be conserved in plasmids QpRS, QpDV, and QpDG, but is absent in a plasmidless isolate in which plasmid sequences have integrated into the chromosomal DNA. Complementation studies demonstrated that fragment C has a plasmid partitioning function and can restore maintenance stability of the partition-defective mini-F plasmid. These data suggest that fragment C carries the plasmid partition region of the plasmid QpH1.

Chromosome partitioning in Escherichia coli in the absence of dam-directed methylation

Escherichia coli dam mutants, lacking the GATC DNA methylase, do not produce anucleate cells at high frequencies, suggesting that hemimethylation of the chromosome origin of replication, oriC, is not essential for correct chromosome partitioning.

Application of the tryptophanase promoter to high expression of the tryptophan synthase gene inEscherichia coli

The application of an inducible regulation system using the tryptophanase operon promoter (TPase promoter; Ptna) was examined for its high expression of the tryptophan synthase (TS) gene in Escherichia coli. The main problem in the application of Ptna for industrial purposes is catabolite repression by glucose, since glucose is the most abundant carbon source. However, this problem could be avoided by changing glucose to an organic acid, such as succinate, fumarate, malate and acetate, in the course of cultivation after glucose initially added was completely consumed. Under these conditions, L-tryptophan was also used to induce tryptophan synthase. Thus, the specific activity of TS in E. coli strain no. 168 harbouring pBR322F-Ptna TS was increased 500-fold compared to that of the cultured host strain. About 1 mol L-tryptophan/l reaction mixture was formed from indole and L-serine at 37 degrees C for 3.5 h.

Partitioning of the F plasmid: overproduction of an essential protein for partition inhibits plasmid maintenance

Multicopy plasmids carrying the sopB gene of the F plasmid inhibit stable inheritance of a coexisting mini-F plasmid. This incompatibility, termed IncG, is found to be caused by excess amounts of the SopB protein, which is essential for accurate partitioning of plasmid DNA molecules into daughter cells. A sopB-carrying multicopy plasmid that shows the IncG+ phenotype was mutagenized in vitro and IncG negative mutant plasmids were isolated. Among these amber and missense mutants of sopB, mutants with a low plasmid copy number and a mutant in the Shine-Dalgarno sequence for translation of the SopB protein were obtained. These results demonstrate that the IncG phenotype is caused by the SopB protein, and that the incompatibility is expressed only when the protein is overproduced. This suggests that the protein must be kept at appropriate concentrations to ensure stable maintenance of the plasmid.

Analysis of the F plasmid centromere

The nucleotide sequence of the cis-acting partition site (centromere) of the miniF plasmid has been determined. Its most notable feature is a reiterated 43 base pair unit. A series of plasmids deleted for portions of the repeat region was constructed and tested for incompatibility with R386 and for stability of inheritance. The extent of incompatibility with R386 was correlated with the number of repeat units. In contrast, the great majority of the repeats were not needed for miniF stability. An adjacent region of unique sequence was also found to be involved in centromere function.

Cell cycle-specific replication of Escherichia coli minichromosomes

The timing of Escherichia coli minichromosome replication in the cell division cycle was examined using an improved procedure for studying plasmid replication frequency. Cultures growing exponentially in glucose/Casamino acids minimal medium were pulse-labeled with [3H]thymidine, and the radioactivity incorporated into plasmid DNA in cells of different ages was analyzed. At the end of the labeling period the bacteria were bound to the surface of a nitrocellulose membrane filter, and the radioactivity in new daughter cells, which eluted continuously from the membrane, was quantitated following agarose gel electrophoresis. The minichromosomes replicated during a discrete interval in the cell division cycle that appeared to coincide with initiation of chromosome replication. In contrast, plasmid pBR322 replicated throughout the division cycle at a rate that increased gradually as a function of cell age. The difference in minichromosome and pBR322 replication was clearly discernible in cells harboring both plasmids. It was also found that the 16 kD gene adjacent to oriC was not a determinant of the timing of minichromosome replication during the division cycle. The results are consistent with the conclusion that minichromosome replication frequency is governed by the same mechanism that controls chromosome replication.

Mechanisms essential for stable inheritance of mini-F plasmid

The F plasmid has its own partition mechanism and ccd mechanism (coupled cell division), besides its own replication mechanism, in order to be stably inherited into daughter cells through cell division. These 3 mechanisms are independent of one another. Therefore, when a DNA segment essential and sufficient for a mechanism is jointed to other heterologous plasmids, the segment is also functional. Most of natural low copy number plasmids might also have their own replication, partition, and ccd mechanisms. These 3 mechanisms may be fundamental to ensure stable inheritance for low copy-number replicons in prokaryotes.

Formation of type II F-primes from unstable Hfrs and their recA-independent conversion to other F-prime types

Four E. coli Hfr strains, representing stable (Hfr Cavalli), moderately stable (AB312) and unstable (Ra-1, Ra-2) Hfr states, were used in the isolation of a series of F' plasmids. Type II F's were found to be the most prevalent F' plasmid formed from all of the Hfrs, while the percentages of delta tra F's increased as the stability of the Hfr increased. Two observations suggested that F' formation in unstable Hfrs like Ra-2 may proceed through a type II F' precursor. First, the major F' products of Ra-2 are tra+ type II F's and, second, other F' types (I, II) and classes (tra+, delta tra) from Ra-2 appeared to be deletion derivatives of a larger F' progenitor. By monitoring the molecular changes that occur when the Ra-2 derived type II F' pWS200 is transferred from one recA host to another, we have found that all F' types and classes can be generated from pWS200 in a recA-independent manner. F sequences involved in the genetic conversions of pWS200 include the oriT locus and the directly repeated gamma delta junctions of F and chromosomal DNA. A model for the formation of F's in unstable Hfrs is postulated in which a tra+ type II F' primary excision product is seen to be modified, through recA-independent processes, to other F' types and classes. This model differs from the current model of F' formation in that independent excision events from the Hfr chromosome are not seen as the source of type I and type II F's. These studies have also shown that the formation of delta tra F's is a recA-independent process that can occur from the F' and Hfr states, that gamma delta-mediated deletions in pWS200 often demonstrate regional specificity in having endpoints near the ilv operon and that genetic alterations in either replication origin of pWS200 (F oriV, chromosomal oriC) stabilize the replication of this "mini-Hfr" cointegrate.

A novel type of E. coli mutants with increased chromosomal copy number

We have isolated E. coli mutants which can grow at 30 degrees C but not at 42 degrees C and are able to harbor the oriC plasmid (minichromosome) at a higher copy number than the parental wild-type strain at the permissive temperature. The mutants were found to contain higher amounts of chromosomal DNA per mg protein than the wild-type, whether or not they harbor the plasmid. Experimental results suggest that the higher amount of chromosomal DNA is due to a higher copy number of chromosomes and not to a larger amount of DNA per chromosome. These properties in each of the mutants are caused by a single mutation at the rpoB or rpoC gene that code for the beta or beta' subunit of RNA polymerase, respectively. The mutations are thought to affect the regulation of replication of oriC-bearing replicons, that is, the E. coli chromosome and oriC plasmids, but not the miniF plasmid.

Cloning and expression of uncI, the first gene of the unc operon of Escherichia coli

The unc operon of Escherichia coli consists of eight genes coding for the eight subunits of the proton-translocating ATPase. In vitro transcription-translation of DNA cloned from the beginning of the operon onto plasmids reveals that the reading frame uncI, which precedes the other genes of the operon, codes for a protein with a molecular weight of 14,500, called i. In minicells, the i protein is synthesized in amounts comparable to the amounts of the ATPase subunits, suggesting that it may be part of the ATPase complex. The presence of the unc promoter and uncI on a plasmid containing the other eight genes of the unc operon has little effect on the differential expression of the unc genes or the partitioning of the newly synthesized subunits into soluble or sedimentable fractions in the in vitro system. The i protein partitions into the sedimentable fraction.

Maintenance and incompatibility of plasmids carrying the replication origin of the Escherichia coli chromosome: evidence for a control region of replication between oriC and asnA

Plasmids that replicate only by means of the cloned Escherichia coli replication origin (oriC) are called minichromosomes or oriC-plasmids. In this paper it is shown that sequences located between oriC and asnA are involved in maintenance and incompatibility of minichromosomes. These sequences include part of the 16kD and 17kD genes, previously allocated within this region (1,2). Transcription towards oriC that is initiated at the 16kD promoter, specifically enhances the stability and copy-number of minichromosomes. Three regions are involved in minichromosome incompatibility. One region, incA, includes the minimal oriC sequence. A second, incB, maps within a 210 base pairs fragment that overlaps the 16kD promoter. The third, incC, encompasses the 17kD gene. Neither one of the regions expresses incompatibility on its own, but the additional presence of one of the others is required. The data presented indicate that sequences of the 16kD and 17kD genes are part of the replication control system of oriC-plasmids.

Mini-F plasmid genes that couple host cell division to plasmid proliferation

A mechanism for stable maintenance of plasmids, besides the replication and partition mechanisms, has been found to be specified by genes of a mini-F plasmid. An oriC plasmid carrying both a mini-F segment necessary for partition [coordinates 46.4-49.4 kilobase pairs (kb) on the F map] and another segment (42.9-43.6 kb), designated ccd (coupled cell division), is more stably maintained than are oriC plasmids carrying only the partition segment; the stability is comparable to that of the parental mini-F plasmid. When replication of a plasmid carrying ccd is prevented and the plasmid copy number decreases, to as few as one per cell, host cell division is inhibited, but not increase of turbidity or chromosome replication. Appearance of plasmid-free segregants is therefore effectively prevented under such conditions. Experimental results suggest that reduction of the copy number of plasmids carrying the ccd region causes an inhibition of cell division and that the ccd region can be dissected into two functional regions; one (ccdB) inhibits cell division and the other (ccdA) releases the inhibition. The interplay of the ccdA and ccdB genes promotes stable plasmid maintenance by coupling host cell division to plasmid proliferation.

Copy number mutations (Cop-) of the plasmid containing the replication origin (oriC) of the Escherichia coli chromosome: lethal effect of the cop region cloned onto a high-copy-number vector on host cells

High-copy-number mutants were isolated from an oriC plasmid. They carried insertion mutations within a region (about 470 base pairs) near the uncB gene. When a segment containing this region was cloned onto a high-copy-number plasmid, such a plasmid could be maintained as an intact form only when it was present in a lower copy number.

Chromosomal replication origins (oriC) of Enterobacter aerogenes and Klebsiella pneumoniae are functional in Escherichia coli

The chromosomal DNA replication origins (oriC) from two members of the family Enterobacteriaceae, Enterobacter aerogenes and Klebsiella pneumoniae, have been isolated as functional replication origins in Escherichia coli. The origins in the SalI restriction fragments of 17.5 and 10.2 kilobase pairs, cloned from E. aerogenes and K. pneumoniae, respectively, were found to be between the asnA and uncB genes, as are the origins of the E. coli and Salmonella typhimurium chromosomes. Plasmids containing oriC from E aerogenes, K. pneumoniae, and S. typhimurium replicate in the E. coli cell-free enzyme system (Fuller, et al., Proc. Natl. Acad. Sci. U.S.A. 78:7370--7374, 1981), and this replication is dependent on dnaA protein activity. These SalI fragments from E. aerogenes and K. pneumoniae carry a region which is lethal to E. coli when many copies are present. We show that this region is also carried on the E. coli 9.0-kilobase-pair EcoRI restriction fragment containing oriC. The F0 genes of the atp or unc operon, when linked to the unc operon promoter, are apparently responsible for the lethality.

Incompatibility of plasmids containing the replication origin of the Escherichia coli chromosome

Plasmids containing the replication origin of the Escherichia coli chromosome (oriC plasmids) are unstable in certain recA strains of E. coli. However, they can be maintained more stably in other recA strains. This stable maintenance has allowed us to study the incompatibility properties of oriC plasmids. We have found that two oriC plasmids are incompatible: they cannot be stably coinherited in individual dividing cells. An oriC plasmid is excluded from growing bacteria at a much faster rate in the presence of a hybrid plasmid made from an oriC plasmid and a high-copy-number vector plasmid than in the presence of another oriC plasmid. By inserting various segments around the oriC region into high-copy-number vectors, we have shown that two different regions in the vicinity of the oriC region determine incompatibility. One region, which we named incA, includes the region essential for autonomous replication of the oriC plasmid. The other, incB, is adjacent to incA but is not required for autonomous replication.

Detection and mapping of six miniF-encoded proteins by cloning analysis of dissected miniF segments

Various DNA subfragments were derived from miniF DNA by complete or partial PstI cleavage, and cloned in the plasmid vectors pBR322 or lambda dv1. The recombinant plasmids obtained were introduced into an Escherichia coli minicell-producing strain, and the plasmid-coded proteins were radiolabeled and analyzed by gel electrophoresis. Six miniF-encoded proteins, larger than 11 000 daltons, were detected and their coding regions were mapped on the F plasmid genome. Three of them were assigned by taking into account the known nucleotide sequences (Murotsu et al. 1981; K. Yoshioka, personal communication). The coding directions of some proteins were determined by inserting the lac promotor into one of the recombinant plasmids and analyzing the increase in production of the proteins. The coding direction of the five proteins analyzed so far was uniform. Comparison of these results with a functional map of miniF suggested possible roles of the proteins.

Identification of a suppressor sequence for DNA replication in the replication origin region of the Bacillus subtilis chromosome

The first replicating fragment of the Bacillus subtilis chromosome, B7, inhibited the replication of the plasmid that carried this fragment. In earlier work using sequential cleavage by Alu I, the suppressor function was located within a 489-base-pair segment. The nucleotide sequence of the entire segment now has been determined. The sequence is characterized by two promoter-like structures and several putative recognition sequences, such as termination signals, 2-fold symmetries, inverted repeats, and repeats. By means of sequential cleavage with exonuclease BAL-31, the essential region for suppression was located in a 200-base-pair region that contains the two promoters with the same orientation. Specific transcription was produced in vitro by using B. subtilis or Escherichia coli RNA polymerases. The transcription was mostly from the second promoter. Elimination of the -35 region of the second promoter dramatically affected both inhibitory activity and in vitro transcription, suggesting that the transcriptional activity of the second promoter is involved in the cis-inhibition of DNA replication. The significance of the suppressor sequence in the region of the replication origin of the B. subtilis chromosome is discussed.

The genes for the eight subunits of the membrane bound ATP synthase of Escherichia coli

The genes for the eight subunits of the membrane bound ATP synthase of Escherichia coli (Ca++, Mg++ dependent ATPase, EC 3.6.1.3) were mapped through genetic, physical and functional analysis of specialized transducing phages lambda asn (von Meyenburg et al. 1978). The ATP synthase genes, designated atp1, are located at 83.2 min in a segment of the chromosome between 3.5 and 11.3 kb left (counterclockwise) of the origin of replication oriC. The counterclockwise order of the genes for the eight subunits, the expression of which starts from a control region at 3.5 kb-L, was found to be: a, (c, b, delta), alpha, gamma, (epsilon, beta) which in the notation of Downie el al. (1981) reads atp B (EFH) A G (C D). The analysis was in part based on the isolation of new types of atp (unc, Suc-) mutations. We made use of the fact that specialized transducing phages lambda asn carrying oriC can establish themselves as minichromosomes rendering asnA cells Asn+, and that the resulting Asn+ cells grow slowly if the lambda asn carries part or all of the atp operon. Selecting for fast growing strains mutations were isolated on the lambda asn which either eliminated atp genes or affected their expression ("promoter" mutations). The relationship between these atp mutations and the cop mutations of Ogura et al. (1980), which also appear to map in front of or within the atp genes, is discussed.

Structure and function of the region of the replication origin of the Bacillus subtilis chromosome. II. Identification of the essential regions for inhibitory functions shown by the DNA segment containing the replication origin

A BamHI restriction endonuclease fragment B7, which contains the replication origin of the Bacillus subtilis chromosome, showed inhibitory effects on cell growth and plasmid replication in Escherichia coli and Bacillus subtilis, when B7 was inserted into a composite plasmid pMS102' and introduced into these cells. In order to localize these properties in more limited regions within the B7 fragment, we developed a new and widely applicable method for deletion of DNA segments of various lengths from one or other end of a given region of the plasmid molecule. Using a set of deletions in the B7 fragments of pMS102'-B7, we determined the loci responsible for the inhibitory effects of B7 as described below. (1) Stickiness appearing in E. coli cells was caused by a segment residing in a region of approximately 2.2 kilobase pairs (kbp) overlapping the E19 and E22 fragments. (2) instability of the plasmid in E. coli was due to a segment localized in the 440 bp region of the E19-side terminal portion of the 2.2 kbp region. (3) The same 440 bp were also responsible for inhibition of the replication of the plasmid in B. subtilis. Hybridization of the cloned DNA fragments containing the 2.2 kbp region with the whole B. subtilis chromosome revealed that several regions of the chromosome are homologous to this characteristic sequence.

Constriction of a fused operon consisting of the recA and kan (kanamycin resistance) genes and regulation of its expression by the lexA gene

The kanamycin resistance gene (kan) of transposon Tn5 was cloned into a derivative of plasmid pBR322. A DNA fragment containing the promoter-operator region of the recA gene was inserted into the promoter region of the cloned kan gene to produce a fused operon, recA-kan. Plasmid pMCR685 carrying recA-kan expressed a low level of activity of the kan gene product (kanamycin phosphotransferase; KPT) in the wild-type cells of Escherichia coli, while the plasmid showed an increased level of the activity in the SPr- mutant cells which produce the inactive lexA protein. The KPT activity in the wild-type cells harboring the plasmid increased 6- to 11-fold upon treatment of the cells with mitomycin C or nalidixic acid, both of which are known to induce synthesis of recA protein. Expression of the recA-kan operon fusion was remarkably repressed by the lexA gene cloned into a plasmid carrying the operon fusion. Higher concentrations of mitomycin C were required for maximal induction of KPT activity in the cells harboring the resulting plasmid pMCR687. These results strongly suggest that the lexA gene product can be itself repress the recA gene, and that pMCR687 is a useful vector to clone genes whose expression is harmful to the host cell growth.

Organization of unc gene cluster of Escherichia coli coding for proton-translocating ATPase of oxidative phosphorylation

The proton-translocating ATPase (F1-F0) of oxidative phosphorylation (ATP phosphohydrolase, EC 3.6.1.3) is coded for by a set of structural genes comprising the unc operon in Escherichia coli. We have analyzed several new transducing phages and plasmids carrying various lengths of the DNA segments of the unc operon by complementation assay using 14 new unc- mutants and representatives of previously described strains which were made available to us. Transducing phages carrying parts of the unc gene cluster were isolated: lambda uncA-9 and lambda glmS phages converted only some of the unc- mutants to the Unc+, as determined by complementation assays. A new hybrid plasmid (pMCR533) carrying part of the unc operon was constructed by inserting the HindIII fragment of lambda asn-5 DNA (a phage carrying the entire unc operon) into the unique HindIII site of pBR322. This plasmid transformed eight unc- strains to Unc+, including uncB402 and uncA401, but did not complement uncD11 or four other strains. Two minichromosomes which carry the E. coli replication origin were also tested: plasmid pNH05 transformed the uncB402 but not the uncA401 strain to Unc+, whereas plasmid pMCF1 transformed none of the mutants tested. Analysis of the DNAs from these transducing phages and plasmids with restriction endonucleases suggested that all of the structural genes for the F1-F0 complex are localized within a DNA segment of approximately 4.5 megadaltons containing two EcoRI sites. The approximate locations of the unc- mutations were mapped on this DNA segment.