Role and specificity of plasmid RP4-encoded DNA primase in bacterial conjugation

Conjugation's Toolkit: the Roles of Nonstructural Proteins in Bacterial Sex

Bacterial conjugation, a form of horizontal gene transfer, relies on a type 4 secretion system (T4SS) and a set of nonstructural genes that are closely linked. These nonstructural genes aid in the mobile lifestyle of conjugative elements but are not part of the T4SS apparatus for conjugative transfer, such as the membrane pore and relaxosome, or the plasmid maintenance and replication machineries. While these nonstructural genes are not essential for conjugation, they assist in core conjugative functions and mitigate the cellular burden on the host. This review compiles and categorizes known functions of nonstructural genes by the stage of conjugation they modulate: dormancy, transfer, and new host establishment. Themes include establishing a commensalistic relationship with the host, manipulating the host for efficient T4SS assembly and function and assisting in conjugative evasion of recipient cell immune functions. These genes, taken in a broad ecological context, play important roles in ensuring proper propagation of the conjugation system in a natural environment.

Probiotics and Prebiotics: Present Status and Future Perspectives on Metabolic Disorders

Metabolic disorders, including type 2 diabetes (T2DM) and cardiovascular disease (CVD), present an increasing public health concern and can significantly undermine an individual's quality of life. The relative risk of CVD, the primary cause of death in T2DM patients, is two to four times higher in people with T2DM compared with those who are non-diabetic. The prevalence of metabolic disorders has been associated with dynamic changes in dietary macronutrient intake and lifestyle changes over recent decades. Recently, the scientific community has considered alteration in gut microbiota composition to constitute one of the most probable factors in the development of metabolic disorders. The altered gut microbiota composition is strongly conducive to increased adiposity, β-cell dysfunction, metabolic endotoxemia, systemic inflammation, and oxidative stress. Probiotics and prebiotics can ameliorate T2DM and CVD through improvement of gut microbiota, which in turn leads to insulin-signaling stimulation and cholesterol-lowering effects. We analyze the currently available data to ascertain further potential benefits and limitations of probiotics and prebiotics in the treatment of metabolic disorders, including T2DM, CVD, and other disease (obesity). The current paper explores the relevant contemporary scientific literature to assist in the derivation of a general perspective of this broad area.

Cholesterol-lowering effects of probiotics and prebiotics: a review of in vivo and in vitro findings

Probiotics are live microorganisms that promote health benefits upon consumption, while prebiotics are nondigestible food ingredients that selectively stimulate the growth of beneficial microorganisms in the gastrointestinal tract. Probiotics and/or prebiotics could be used as alternative supplements to exert health benefits, including cholesterol-lowering effects on humans. Past in vivo studies showed that the administration of probiotics and/or prebiotics are effective in improving lipid profiles, including the reduction of serum/plasma total cholesterol, LDL-cholesterol and triglycerides or increment of HDL-cholesterol. However, other past studies have also shown that probiotics and prebiotics had insignificant effects on lipid profiles, disputing the hypocholesterolemic claim. Additionally, little information is available on the effective dosage of probiotics and prebiotics needed to exert hypocholesterolemic effects. Probiotics and prebiotics have been suggested to reduce cholesterol via various mechanisms. However, more clinical evidence is needed to strengthen these proposals. Safety issues regarding probiotics and/or prebiotics have also been raised despite their long history of safe use. Although probiotic-mediated infections are rare, several cases of systemic infections caused by probiotics have been reported and the issue of antibiotic resistance has sparked much debate. Prebiotics, classified as food ingredients, are generally considered safe, but overconsumption could cause intestinal discomfort. Conscientious prescription of probiotics and/or prebiotics is crucial, especially when administering to specific high risk groups such as infants, the elderly and the immuno-compromised.

Escherichia coli genes affecting recipient ability in plasmid conjugation: are there any?

Background

How does the recipient cell contribute to bacterial conjugation? To answer this question we systematically analyzed the individual contribution of each Escherichia coli gene in matings using plasmid R388 as a conjugative plasmid. We used an automated conjugation assay and two sets of E. coli mutant collections: the Keio collection (3,908 E. coli single-gene deletion mutants) and a collection of 20,000 random mini-Tn10::Km insertion mutants in E. coli strain DH5α. The combined use of both collections assured that we screened > 99% of the E. coli non-essential genes in our survey.

Results

Results indicate that no non-essential recipient E. coli genes exist that play an essential role in conjugation. Mutations in the lipopolysaccharide (LPS) synthesis pathway had a modest effect on R388 plasmid transfer (6 – 32% of wild type). The same mutations showed a drastic inhibition effect on F-plasmid transfer, but only in liquid matings, suggesting that previously isolated conjugation-defective mutants do in fact impair mating pair formation in liquid mating, but not conjugative DNA processing or transport per se.

Conclusion

We conclude from our genome-wide screen that recipient bacterial cells cannot avoid being used as recipients in bacterial conjugation. This is relevant as an indication of the problems in curbing the dissemination of antibiotic resistance and suggests that conjugation acts as a pure drilling machine, with little regard to the constitution of the recipient cell.

Plasmid donor affects host range of promiscuous IncP-1beta plasmid pB10 in an activated-sludge microbial community

Horizontal transfer of multiresistance plasmids in the environment contributes to the growing problem of drug-resistant pathogens. Even though the plasmid host cell is the primary environment in which the plasmid functions, possible effects of the plasmid donor on the range of bacteria to which plasmids spread in microbial communities have not been investigated. In this study we show that the host range of a broad-host-range plasmid within an activated-sludge microbial community was influenced by the donor strain and that various mating conditions and isolation strategies increased the diversity of transconjugants detected. To detect transconjugants, the plasmid pB10 was marked with lacp-rfp, while rfp expression was repressed in the donors by chromosomal lacI(q). The phylogeny of 306 transconjugants obtained was determined by analysis of partial 16S rRNA gene sequences. The transconjugants belonged to 15 genera of the alpha- and gamma-Proteobacteria. The phylogenetic diversity of transconjugants obtained in separate matings with donors Pseudomonas putida SM1443, Ralstonia eutropha JMP228, and Sinorhizobium meliloti RM1021 was significantly different. For example, the transconjugants obtained after matings in sludge with S. meliloti RM1021 included eight genera that were not represented among the transconjugants obtained with the other two donors. Our results indicate that the spectrum of hosts to which a promiscuous plasmid transfers in a microbial community can be strongly influenced by the donor from which it transfers.

Mechanisms of strand replacement synthesis for plasmid DNA transferred by conjugation

A single strand of plasmid DNA is transferred during conjugation. We examined the mechanism of complementary strand synthesis in recipient cells following conjugative mobilization of derivatives of the IncQ plasmid R1162. A system for electroporation of donor cells, followed by immediate mating, was used to eliminate plasmid-specific replicative functions. Under these conditions, Escherichia coli recipients provided a robust mechanism for initiation of complementary strand synthesis on transferred DNA. In contrast, plasmid functions were important for efficient strand replacement in recipient cells of Salmonella enterica serovar Typhimurium. The mobilizing vector for R1162 transfer, the IncP1 plasmid R751, encodes a DNA primase with low specificity for initiation. This protein increased the frequency of transfer of R751 into Salmonella, but despite its low specificity, it was inactive on the R1162 derivatives. The R751 primase was slightly inhibitory for the transfer of both R751 and R1162 into E. coli. The results show that there is a chromosomally encoded mechanism for complementary strand synthesis of incoming transferred DNA in E. coli, while plasmid-specific mechanisms for this synthesis are important in Salmonella.

Genetic evidence of protein transfer during bacterial conjugation

DNA can be transferred from eubacteria to at least plants, fungi, and all other eubacteria by related, plasmid-mediated conjugation. Little is known about the biochemistry of intraspecies or interspecies DNA transfer. Even less is known about what other molecules may accompany the DNA, or the direct or inheritable effects on recipients of these escort molecules. This report describes a genetic assay for detecting protein transfer during conjugation. The assay monitored phage lambda released from lysogenic recipients as a result of the concomitant delivery of the Escherichia coli RecA protein and plasmid DNA. The heretofore unexpected transfer of a donor chromosome-encoded protein initiates a heritable change in the recipient without altering its genetic make-up. The mechanism of transfer could be independent of transferred DNA.

IncP plasmids are unusually effective in mediating conjugation of Escherichia coli and Saccharomyces cerevisiae: involvement of the tra2 mating system

Mobilizable shuttle plasmids containing the origin-of-transfer (oriT) region of plasmids F (IncFI), ColIb-P9 (IncI1), and RP4/RP1 (IncPalpha) were constructed to test the ability of the cognate conjugation system to mediate gene transfer from Escherichia coli to Saccharomyces cerevisiae. Only the Palpha system caused detectable mobilization to yeast, giving peak values of 5 x 10(-5) transconjugants per recipient cell in 30 min. Transfer of the shuttle plasmid required carriage of oriT in cis and the provision in trans of the Palpha Tra1 core and Tra2 core regions. Genes outside the Tra1 core did not increase the mobilization efficiency. All 10 Tra2 core genes (trbB, -C, -D, -E, -F, -G, -H, -I, -J, and -L) required for plasmid transfer to E. coli K-12 were needed for transfer to yeast. To assess whether the mating-pair formation (Mpf) system or DNA-processing apparatus of the Palpha conjugation system is critical in transkingdom transfer, an assay using an IncQ-based shuttle plasmid specifying its own DNA-processing system was devised. RP1 but not ColIb mobilized the construct to yeast, indicating that the Mpf complex determined by the Tra2 core genes plus traF is primarily responsible for the remarkable fertility of the Palpha system in mediating gene transfer from bacteria to eukaryotes.

Requirement of a limited segment of the sog gene for plasmid R64 conjugation

The sog gene of the IncI1 plasmid R64 was sequenced and characterized. The sog gene was shown to express two acidic proteins, SogL and SogS, with 1255 and 844 amino acid residues, respectively. The SogS protein was expressed by translational reinitiation within the SogL reading frame. Analysis of dnaG-suppression activity using the Escherichia coli dnaG strain indicated that the domain for this activity was located within the N-terminal one-third segment of the SogL protein. A Deltasog mutation was constructed by replacing most of the sog coding sequence with a DNA fragment encoding a tetracycline resistance gene. Introduction of the Deltasog mutation into an R64 derivative resulted in approximately a 50-fold reduction in transfer frequency. It was observed that only a limited portion of the SogL or SogS protein corresponding to an internal 0.94-kb EcoRV-SnaBI segment of the sog gene was required for the conjugal transfer of R64.

Identification and characterization of RP1 Tra1 cistrons involved in pilus function and plasmid mobilization

Transfer-defective mutants of the Tra1 region of RP1 were isolated. Complementation studies involving stable heterozygotes combined with the mapping of Tn5 insertion mutations revealed two pilus cistrons, pilA and pilB, at positions 46.9 to 48.2 kb and 46.0 to 46.4 kb, respectively. All pilB mutants were Dps- (i.e., resistant to donor-specific phages PR4 and PRR1), whereas pilA mutants were Dps- (promoter-proximal mutations), Dps+/- (sensitive only to PR4 [more centrally located mutations]), or Dps+ (sensitive to both phages [promoter-distal mutations]). The correlation between the site mutated and the Dps phenotype, together with the finding that certain Dps+ pilA mutants continued to mobilize nonconjugative plasmids, suggested that pilA is bifunctional, contributing both to pilus function (at the promoter-proximal end) and to RP1 mobilization. It was also shown that the 43.5- to 49.5-kb region that includes pilA and pilB encodes all of the Tra1 pilus functions required for propagation of donor-specific phages and hence, probably, for pili that are active in conjugation. Finally, three cistrons that specifically affect RP1 mobilization were identified. Two of these, mobA and mobB, occur immediately anticlockwise to oriT and probably correspond to the traJ and traI genes characterized by other workers. The third cistron, mobC, occurs clockwise to oriT and may be a new mobilization gene, since its function can be substituted by IncP beta plasmids, a feature different from that of the traK mobilization gene which occurs in the same region but is RP1 specific. None of the mob cistrons was required for mobilization of nonconjugative plasmids, except for mobB, which was required by pVS99.

The divergent promoters mediating transcription of the par locus of plasmid RP4 are subject to autoregulation

The partitioning region of broad-host-range plasmid RP4 contains four genes (parA, parB, parC, and parD) that encode products essential for partition activity. Two divergently arranged promoters located in the intercistronic region between parC and parD mediate transcription of these genes. The transcriptional initiation sites for both promoters were determined by primer extension. Transcriptional fusions were used to show that parA, parB, and parC are combined in an operon, while parD constitutes a separate transcription unit. Both parCBA (genes in order of transcription) and parD are negatively autoregulated at the level of transcription by the gene products of parA and parD, respectively. parD promoter mutants which have become insensitive to repression by parD were isolated. Comparison of wild type and the mutant parD promoter sequences indicated that three short repeats are likely involved in the negative regulation of this promoter. Potentially these sequence elements comprise target sites for the ParD protein.

Characterization of a Tra 2 function of RP1 that affects growth of Pseudomonas aeruginosa PAO and surface exclusion in Escherichia coli K12

pVS438, a clone of part of the Tra 2 region of RP1 in RSF1010, confers two unusual phenotypes: poor growth (Slo+) in Pseudomonas aeruginosa PAO and surface exclusion (Sfx+) in Escherichia coli K12. Both of these phenotypes were found to be encoded by a 1.8-kb fragment of RP1 (from 25.9-27.7 kb) that spans the traB gene. However, whether both phenotypes, neither, or only Slo+ is expressed by this fragment depends on its location and orientation in RSF1010. In pVS438, where this fragment occurs in the SmR locus of RSF1010, expression of the Sfx+ phenotype is due to augmented transcription from the two promoters that cotranscribe the SuRSmR genes. When augmentation is abolished by insertion of Tn5 between these promoters and the cloned fragment, or by insertion of the fragment elsewhere in RSF1010, a Slo+Sfx- phenotype results. DNA that confers only the Slo+ phenotype was mapped to the 26.2-26.8 kb region of RP1 between traE and traB and the designation, traS, given to the gene responsible. Despite the recognition of a traS+ (Slo+) component of DNA within that encoding the Slo+ and Sfx+ phenotypes, this gene seems nevertheless to be responsible for the Sfx+ phenotype since hydroxylamine-induced Slo- mutants of pVS438 are usually also Sfx-. These apparently conflicting observations and the precise interplay between the Slo+, Sfx+, and TraB+ phenotypes were not resolved. Finally, traS is not essential for plasmid transfer since pVS438 and a Slo-Sfx- derivative of it can both equally complement an RP1tra-deletion mutant of part of the Tra 2 region.

Gene organization and nucleotide sequence of the primase region of IncP plasmids RP4 and R751

The primase genes of RP4 are part of the primase operon located within the Tra1 region of this conjugative plasmid. The operon contains a total of seven transfer genes four of which (traA, B, C, D) are described here. Determination of the nucleotide sequence of the primase region confirmed the existence of an overlapping gene arrangement at the DNA primase locus (traC) with in-phase translational initiation signals. The traC gene encodes two acidic and hydrophilic polypeptide chains of 1061 (TraC1) and 746 (TraC2) amino acids corresponding to molecular masses of 116,721 and 81,647 Da. In contrast to RP4 the IncP beta plasmid R751 specifies four large primase gene products (192, 152, 135 and 83 kDa) crossreacting with anti-RP4 DNA primase serum. As shown by deletion analysis at least the 135 and 83 kDa polypeptides are two separate translational products that by analogy with the RP4 primases, arise from in-phase translational initiation sites. Even the smallest primase gene products TraC2 (RP4) and TraC4 (R751) exhibit primase activity. Nucleotide sequencing of the R751 primase region revealed the existence of three in-phase traC translational initiation signals leading to the expression of gene products with molecular masses of 158,950 Da, 134,476 Da, and 80,759 Da. The 192 kDa primase polypeptide is suggested to be a fusion protein resulting from an in frame translational readthrough of the traD UGA stopcodon. Distinct sequence similarities can be detected between the TraC proteins of RP4 and R751 gene products TraC3 and TraC4 and in addition between the TraD proteins of both plasmids. The R751 traC3 gene contains a stretch of 507 bp which is unrelated to RP4 traC or any other RP4 Tra1 gene.

Genetic characterization of the stabilizing functions of a region of broad-host-range plasmid RK2

One of the regions responsible for the stable inheritance of the broad-host-range plasmid RK2 is contained within the PstI C fragment, located from coordinates 30.8 to 37.0 kb (P.N. Saurugger, O. Hrabak, H. Schwab, and R.M. Lafferty, J. Biotechnol. 4:333-343, 1986). Genetic analysis of this 6.2-kb region demonstrated that no function was present that stabilized by selectively killing plasmid-free segregants. The sequence from 36.0 to 37.0 kb mediated a twofold increase in plasmid copy number, but this region was not required for stabilization activity. The PstI C fragment was shown to encode a multimer resolution system from 33.1 to 35.3 kb. The resolution cis-acting site was mapped to 140 bp, sequenced, and observed to contain two directly repeated sequences of 6 and 7 bases and two perfect inverted repeats of 6 and 8 bases. The trans-acting factor(s) was mapped and functionally determined to encode a resolvase capable of catalyzing recombination at high frequency between cis-acting sites in either direct or inverted orientation. Multimer resolution alone did not account for complete plasmid stabilization by the PstI C fragment, since removal of regions adjacent to the 35.3-kb border of the minimal mrs locus dramatically reduced stabilization. The minimal region required for complete stabilization, from 32.8 to 35.9 kb, was capable of fully stabilizing plasmids independently of the replicon or the recA proficiency of the host. Stabilization activity was also fully expressed in several diverse gram-negative bacteria, whereas the F plasmid par locus functioned only in Escherichia coli. On the basis of these observations, we conclude that under the growth conditions used, the minimal stabilization locus encodes both an mrs activity and a stabilization activity that has the properties of a par locus.

Protein transfer into the recipient cell during bacterial conjugation: studies with F and RP4

Transfer of donor cell proteins to the recipient bacterium was examined in F- and RP4-mediated conjugation. Transfer of a 120 kD polypeptide, identified as the larger product of the plasmid DNA primase gene, was readily detected during RP4-promoted conjugation. The protein was transmitted to the cytoplasm of the recipient, presumably complexed to the transferred ssDNA. F DNA was transferred without detectable association with any cytoplasmic tra protein or with the ssDNA-binding protein encoded by the plasmid. However, a 92 kD protein, possibly F TraD product, was transmitted to the membrane fraction of the recipient cell.

Initiation of DNA synthesis in the transfer origin region of RK2 by the plasmid-encoded primase: detection using defective M13 phage

The broad host range IncP (IncP1) plasmids of gram-negative bacteria encode DNA primases that are involved in conjugal DNA synthesis. The primase of RK2/RP4 is required for efficient DNA transfer to certain gram-negative bacteria, indicating that the enzyme primes complementary strand synthesis in the recipient. In vitro, the primase initiates synthesis of oligoribonucleotides at 3'-dGdT-5' dinucleotides on the template strand. In this report, replication-defective M13 phage are used to assay the ability of the RK2-encoded primase to initiate complementary strand synthesis in vivo on single-strand templates containing the RK2 origin of conjugal transfer (oriT) or the RK2 origin of vegetative replication (oriV). The results show that sequences from either strand of the oriT region serve as efficient substrates for the RK2 primase and can enhance the growth of the defective M13 vectors delta E101 and delta Elac to levels approaching wild-type. The primise-oriT interaction appeared specific, since neither the oriV sequence nor another RK2 region, trfB, significantly enhanced growth of the defective phage, either in the presence or in the absence of the primase. In contrast to ColEl and F, this study also shows that the oriV region of RK2 lacks sites that are recognized by the host-specified DNA priming systems. The results suggest that the oriT region contains sites on both DNA strands that are efficient substrates for the plasmid-encoded primase, facilitating initiation of complementary strand DNA synthesis in both donor and recipient during conjugation.

Replication of plasmids in gram-negative bacteria

Replication of plasmid deoxyribonucleic acid (DNA) is dependent on three stages: initiation, elongation, and termination. The first stage, initiation, depends on plasmid-encoded properties such as the replication origin and, in most cases, the replication initiation protein (Rep protein). In recent years the understanding of initiation and regulation of plasmid replication in Escherichia coli has increased considerably, but it is only for the ColE1-type plasmids that significant biochemical data about the initial priming reaction of DNA synthesis exist. Detailed models have been developed for the initiation and regulation of ColE1 replication. For other plasmids, such as pSC101, some hypotheses for priming mechanisms and replication initiation are presented. These hypotheses are based on experimental evidence and speculative comparisons with other systems, e.g., the chromosomal origin of E. coli. In most cases, knowledge concerning plasmid replication is limited to regulation mechanisms. These mechanisms coordinate plasmid replication to the host cell cycle, and they also seem to determine the host range of a plasmid. Most plasmids studied exhibit a narrow host range, limited to E. coli and related bacteria. In contrast, some others, such as the IncP plasmid RK2 and the IncQ plasmid RSF1010, are able to replicate in nearly all gram-negative bacteria. This broad host range may depend on the correct expression of the essential rep genes, which may be mediated by a complex regulatory mechanism (RK2) or by the use of different promoters (RSF1010). Alternatively or additionally, owing to the structure of their origin and/or to different forms of their replication initiation proteins, broad-host-range plasmids may adapt better to the host enzymes that participate in initiation. Furthermore, a broad host range can result when replication initiation is independent of host proteins, as is found in the priming reaction of RSF1010.

Two mechanisms necessary for the stable inheritance of plasmid RP4

Plasmid RP4 is stably maintained in strains of Escherichia coli and other Gram-negative bacteria. Inactivation of the plasmid primase gene (pri) or removal of the PstIC fragment gave RP4 derivatives that are slightly unstably maintained in E. coli. Removal of the Tn 1 multimer resolution system (res and tnpR) did not lead to any detectable plasmid loss. Removal of all three of these regions, however, gave rise to pNJ5000 which is lost at high frequency. We have dissected the mechanisms causing this phenomenon. In contrast to RP4, pNJ5000 accumulates significantly as plasmid multimers in a Rec+ host; in a recA host, multimers are not seen and the plasmid is stably maintained. Multimers therefore appear to form by recA-mediated homologous recombination and cause plasmid instability, perhaps by interfering with partition. We demonstrate a mechanism provided by the PstIC fragment which acts on multimers analogously to the Tn1/3 resolution system on plasmid cointegrates, being effective only when cloned in cis. The loss of pri, on the other hand, can be complemented in trans. Our results are consistent with the view that primase prevents multimers forming (rather than resolving them once formed), perhaps by binding specifically to single-stranded regions of the plasmid and preventing homologous pairing.

Mutagenesis of the Tra1 core region of RK2 by using Tn5: identification of plasmid-specific transfer genes

The conjugation system of the IncP alpha plasmid RK2/RP4 is encoded by transfer regions designated Tra1, Tra2, and Tra3. The Tra1 core region, cloned on plasmid pDG4 delta 22, consists of the origin of transfer (oriT) and 2.6 kilobases of flanking DNA providing IncP alpha plasmid-specific functions that allow pDG4 delta 22 to be mobilized by the heterologous IncP beta plasmid R751. Tn5 insertions in pDG4 delta 22 define a minimal 2.2-kilobase region required for plasmid-specific transfer of oriT. The Tra1 core contains the traJ and traK genes as well as an 18-kilodalton open reading frame downstream of traJ. The traJ and traK genes were shown to be required for transfer by complementation of inserts within these genes. Genetic evidence for the role of the 18-kilodalton open reading frame in transfer was obtained, although this protein has not been detected in cell lysates. These studies indicate that at least three transfer proteins are involved in plasmid-specific interactions at oriT.

Molecular genetic analysis of bacterial plasmid promiscuity

The molecular genetic basis of the promiscuity of the wide host range conjugative IncP-1 alpha plasmids has been investigated by transposon mutagenesis and by the construction of minireplicons. The former has identified the origin of plasmid vegetative replication, the replication genes needed for initiation of plasmid replication, the DNA primase gene and a gene encoding a polypeptide of 52 kDa and mapping near the origin of plasmid transfer as all contributing to promiscuity. Minireplicon constructions confirm this conclusion but in addition establish that the origins of replication, transfer and other genomic regions produce complex interactions with respect to host range. DNA sequence analysis within the origin of replication show that the first direct repeat of the cluster of five repeats and sequences immediately 5' to it appear to be required in some (Escherichia coli) but not in other (Pseudomonas aeruginosa) hosts for plasmid replication.

Molecular cloning of the plasmid RP4 primase region in a multi-host-range tacP expression vector

Plasmid RP4 primase was overproduced by utilizing autoregulated high-level expression vector systems in Escherichia coli and in four other Gram-negative bacterial species. Analysis of the products in E. coli revealed that in addition to the two primase polypeptides of 118 and 80 kDa the pri region of RP4 encodes two smaller proteins of 16.5 and 8.6 kDa. The transcript for the four RP4-specified products is polycistronic. The vector system used in E. coli is based on the plasmid pKK223-3 (Brosius and Holy, 1984), a ColE1-type replicon which contains a polylinker sequence flanked on one side by the controllable tac promoter and on the other side by two strong transcriptional terminators. The gene for the lac repressor (lacIQ) was inserted to render the use of the plasmid independent from repressor-overproducing strains. The gene cartridge essential for high-level expression and selection was combined with the RSF1010 replicon to generate a vector plasmid functioning in a wide variety of Gram-negative hosts. The versatility of the vector family was extended by constructing derivatives that contain the polylinker in inverted orientation relative to the tac promoter. Therefore, the orientation of the cloned fragment can be chosen by 'forced cloning' into the appropriately selected vector.