pT181 plasmid replication is regulated by a countertranscript-driven transcriptional attenuator

Mechanism of binding of the antisense and target RNAs involved in the regulation of IncB plasmid replication

The replication frequency of the IncB miniplasmid pMU720 is dependent upon the expression of the repA gene. Binding of a small, highly structured, antisense RNA (RNA I) to its complementary target in the RepA mRNA (RNA II) inhibits repA expression and thus regulates replication. Analyses of binding of RNA I to RNA II indicated that the reaction consists of three major steps. The first step, initial kissing complex formation, involves base pairing between complementary sequences in the hairpin loops of RNA I and RNA II. The second step is facilitated by interior loop structures in the upper stems of RNA I and RNA II and involves intrastand melting and interstrand pairing of the upper stem regions to form an extended kissing complex. This complex was shown to be sufficient for inhibition of repA expression. The third step involves stabilization of the extended kissing complex by pairing between complementary single-stranded tail regions of RNA I and RNA II. Thus, the final product of RNA I-RNA II binding is not a full duplex between the two molecules.

Plasmids of the pT181 family show replication-specific initiator protein modification

The rolling circle plasmids of Staphylococcus aureus regulate their replication by controlling initiator (Rep) protein synthesis. It was demonstrated recently that the pT181 initiator protein RepC is inactivated during pT181 replication by the addition of an oligodeoxynucleotide, giving rise to a new form, RepC* (A. Rasooly and R. P. Novick, Science, 262:1048-1050). We establish here that this initiator modification occurs with four other members of the pT181 family and that it occurs in Bacillus subtilis as well as S. aureus. These results suggest that Rep conversion to Rep* is probably universal among plasmids of the pT181 family and is not host dependent.

Control of replication of the Lactobacillus pentosus plasmid p353-2: evidence for a mechanism involving transcriptional attenuation of the gene coding for the replication protein

The synthesis of plasmid DNA and of RNA encoded by the replication protein gene (rep) of plasmid p353-2 of Lactobacillus pentosus was studied for the wild-type plasmid and for a mutant plasmid with a deletion in the 5' untranslated region of the rep gene. Plasmid p353-2 codes for two countertranscript RNAs (CT-RNA) of approximately 75 and 250 nucleotides transcribed from the 5' untranslated region of the rep gene, in opposite directions. In a mutant plasmid with a deletion of the promoter and part of the CT-RNA-encoding sequence which shows a 5- to 10-fold increase in copy number compared to the wild-type plasmid, no CT-RNA could be detected. In the wild-type plasmid more than 90% of transcription initiated at a promoter upstream of the rep gene is prematurely terminated to form a 190 nucleotide truncated RNA, whereas in the mutant plasmid nearly all transcripts reach a size (1100 nucleotides) corresponding to that of the rep gene. A model is presented for the role of CT-RNA in control of plasmid replication, similar to that previously presented for the staphylococcal plasmid pT181, involving a mechanism of transcriptional attenuation of rep RNA at a site just upstream of the rep gene.

Genetics of lactobacilli: plasmids and gene expression

This paper reviews the present knowledge of the structure and properties of small (< 5 kb) plasmids present in Lactobacillus spp. The data show that plasmids from Lactobacillus spp., like many plasmids from other Gram-positive bacteria, display a modular organization and replicate by a mechanism of rolling circle replication. Structurally, plasmids from lactobacilli are closely related to plasmids from other Gram-positive bacteria. They contain elements (plus- and minus origin of replication, element(s) for control of plasmid replication, mobilization function) showing extensive similarity to analogous elements in plasmids from these other organisms. It is believed that lactobacilli have acquired such elements by intra- and/or intergenic transfer mechanisms. The first part of the review is concluded with a description of plasmid vectors with a Lactobacillus replicon and integrative vectors, including data concerning their structural and segregational stability. In the second part of this review we describe the progress that has been made during the last few years in identifying and characterizing elements that control expression of genetic information in lactobacilli. Based on the sequence of eleven identified and twenty presumed promoters, some preliminary conclusions can be drawn regarding the structure of Lactobacillus promoters. A typical Lactobacillus promoter shows significant similarity to promoters from E. coli and B. subtilis. An analysis of published sequences of seventy genes indicates that the region encompassing the translation start codon AUG also shows extensive similarity to that of E. coli and B. subtilis. Codon usage of Lactobacillus genes is not random and shows interspecies as well as intraspecies heterogeneity. Interspecies differences may, in part, be explained by differences in G+C content of different lactobacilli. Differences in gene expression levels can, to a large extent, account for intraspecies differences of codon usage bias. Finally, we review the knowledge that has become available concerning protein secretion and heterologous gene expression in lactobacilli. This part is concluded with a compilation of data on the expression in Lactobacillus of heterologous genes under the control of their own promoter or under control of a Lactobacillus promoter.

Control of ColE2 DNA replication: in vitro binding of the antisense RNA to the Rep mRNA

Expression of the Rep protein of colicin E2 plasmid is negatively controlled at a post-transcriptional step by a plasmid-coded RNA (RNAI). RNAI is complementary to the 5' nontranslated region of the Rep mRNA and can be folded into two stem-loop structures. Several cop mutations that increased the copy number of the plasmid have been mapped in the RNAI coding region. Here we demonstrated that RNAI and the Rep mRNA rapidly form a stable complex under near physiological conditions. The cop mutations drastically reduced the binding rate. The reduction correlated to the in vivo phenotypes of the cop mutant plasmids, indicating that binding of RNAI to the Rep mRNA is involved in the regulation of the Rep protein expression. Binding properties of RNAI and the Rep mRNA with the cop mutations and those with various extents of deletion suggested that the initial interaction of the two RNAs occurs between the loop portions of the larger of the two stem-loop structures. RNAI does not completely sequester the putative S/D region, even when the two RNAs form a complete duplex. Possible mechanisms of the inhibition of the rep gene expression by binding of RNAI to the Rep mRNA under such a situation were discussed.

Characterization of the Staphylococcus aureus chromosomal gene pcrA, identified by mutations affecting plasmid pT181 replication

The Staphylococcus aureus chromosomal gene pcrA, identified by mutations, such as pcrA3, that affect plasmid pT181 replication, has been cloned and sequenced. The pcrA gene encodes a protein with significant similarity (40% identity) to two Escherichia coli helicases: the helicase II encoded by the uvrD gene and the Rep helicase. The pcrA3 mutation was found to be a C to T transition leading to a threonine to isoleucine substitution at amino acid residue 61 of the protein. The pcrA gene seems to belong to an operon containing at least one other gene, tentatively named pcrB, upstream from pcrA. The PcrA protein was shown to be essential for cell viability and overproduction has deleterious effects on the host and plasmid replication.

Mutations affecting pseudoknot control of the replication of B group plasmids

The translational initiation region of the mRNA for the replication initiation protein (RepA) of pMU720 is predicted to be sequestered in an inhibitory secondary structure designated stem-loop III. Activation of repA translation requires both the disruption of stem-loop III by ribosomes involved in the translation and termination of the leader peptide RepB and the formation of a pseudoknot, a tertiary RNA structure. Disruption of stem-loop III by site-directed mutagenesis was found to be insufficient to allow high repA expression in the absence of pseudoknot formation, indicating that the pseudoknot acts as an enhancer of repA translation. Furthermore, extending the length of the leader peptide RepB and changing the distance between the pseudoknot and repA Shine-Dalgarno sequence were found to have major effects on the translation of repA.

Mechanisms of genome propagation and helper exploitation by satellite phage P4

Temperate coliphage P2 and satellite phage P4 have icosahedral capsids and contractile tails with side tail fibers. Because P4 requires all the capsid, tail, and lysis genes (late genes) of P2, the genomes of these phages are in constant communication during P4 development. The P4 genome (11,624 bp) and the P2 genome (33.8 kb) share homologous cos sites of 55 bp which are essential for generating 19-bp cohesive ends but are otherwise dissimilar. P4 turns on the expression of helper phage late genes by two mechanisms: derepression of P2 prophage and transactivation of P2 late-gene promoters. P4 also exploits the morphopoietic pathway of P2 by controlling the capsid size to fit its smaller genome. The P4 sid gene product is responsible for capsid size determination, and the P2 capsid gene product, gpN, is used to build both sizes. The P2 capsid contains 420 capsid protein subunits, and P4 contains 240 subunits. The size reduction appears to involve a major change of the whole hexamer complex. The P4 particles are less stable to heat inactivation, unless their capsids are coated with a P4-encoded decoration protein (the psu gene product). P4 uses a small RNA molecule as its immunity factor. Expression of P4 replication functions is prevented by premature transcription termination effected by this small RNA molecule, which contains a sequence that is complementary to a sequence in the transcript that it terminates.

RepR protein expression on plasmid pIP501 is controlled by an antisense RNA-mediated transcription attenuation mechanism

Expression of the rate-limiting initiator protein RepR of plasmid pIP501 is controlled by the antisense RNAIII. Mutational alteration of individual G residues within the single-stranded loops of RNAIII led to an increase in copy number. In contrast to the G-rich single-stranded loops, two smaller AT-rich loops of RNAIII were found to be dispensable for its inhibitory function. Reciprocal mutations in the same loop compensated for each other's effect, and a destabilization of the major stem structure of RNAIII also resulted in an increased copy number. These data were consistent with the idea that the interaction of RNAIII with its target starts with the formation of a kissing complex between the single-stranded loops of both molecules. The repR mRNA leader sequence, which includes the target of RNAIII, is able to assume two alternative structures due to the presence of two inverted repeats the individual sequences of which are mutually complementary. In the presence of the antisense RNAIII, one of these inverted repeats (IR2) is forced to fold into a transcriptional terminator structure that prevents transcription of the repR gene. In the absence of RNAIII, formation of the transcriptional terminator is prevented and expression of the essential repR gene can proceed normally. This antisense RNA-driven transcriptional attenuation mechanism was supported by extensive deletional analysis and direct evidence that IR2 functions as a transcriptional terminator.

Interaction between the antisense and target RNAs involved in the regulation of IncB plasmid replication

Physical analysis of RNA I, the small antisense RNA which regulates the replication of IncB miniplasmid pMU720, showed that it is a highly structured molecule containing an imperfectly paired stem closed by a 6-base hairpin loop. Mutational studies revealed that a 3-base sequence in the hairpin loop is critical to the interaction between RNA I and its complementary target in the RepA mRNA (RNA II). Furthermore, a 2-base interior loop in the upper stem was found to play an important role in facilitating effective binding between RNA I and RNA II. From these analyses, a model describing the molecular mechanism of binding between RNA I and RNA II is proposed.

Regulation of the Staphylococcus aureus plasmid pI258 mercury resistance operon

Experiments involving fusion between the Staphylococcus aureus plasmid pI258-encoded mer operon and the reporter gene beta-lactamase, mutational analysis, and trans-complementation studies have shown that the merR gene of pI258, which shows DNA sequence similarity with known merR genes from other bacteria, regulates the expression of the mer operon in vivo. The merR gene product is a trans-acting protein that activates mer operon transcription in the presence of the inducers Hg2+ and Cd2+. A glutathione-S-transferase-MerR fusion protein specifically bound and protected a 27-nucleotide operator sequence from DNase I digestion. This operator sequence is highly homologous with mer operator sequences of other known systems.

Bacteriophage P4 immunity controlled by small RNAs via transcription termination

Satellite bacteriophage P4 immunity is encoded within a short DNA region 357 bp long containing the promoter PLE and 275 bp downstream. PLE is active both in the early post-infection phase, when genes necessary for P4 lytic cycle are transcribed from this promoter, and in the lysogenic condition, when expression of the above genes is prevented by prophage immunity. In order to understand how P4 immunity is elicited, we have characterized the transcription pattern during the establishment and the maintenance of the satellite phage P4 lysogenic condition. We found that prophage transcription starting at PLE ends prematurely and the transcripts do not extend beyond 300-400 nucleotides downstream of PLE. Thus P4 immunity acts by causing premature transcription termination rather than by repressing transcription initiation. The P4 immunity region is transcribed in the prophage, but it does not seem to be translated; this region contains two elements (seqA and seqB) of a palindromic sequence. In addition to transcripts about 300 nucleotides long, P4 prophage produces a family of shorter transcripts, about 80 nucleotides long, containing seqA or seqB. Evidence is presented suggesting that SeqB RNA is the trans-acting immunity factor, and that interaction of SeqB RNA with the complementary nascent RNA containing seqA may be involved in bringing about premature transcription termination.

Structural and functional analysis of two cryptic plasmids from Lactobacillus pentosus MD353 and Lactobacillus plantarum ATCC 8014

The DNA sequences of a 2.4 kb plasmid (p353-2) from Lactobacillus pentosus MD353 and a 1.9 kb plasmid (p8014-2) from Lactobacillus plantarum ATCC 8014 show 81.5% overall similarity. Both plasmids carry elements (replication protein gene, plus-origin and minus-origin of replication), which are typical of plasmids that replicate via a rolling-circle mechanism of replication (RCR). Direct evidence for an RCR mechanism was obtained by showing the accumulation of single-stranded plasmid intermediates in the presence of rifampicin. A minus-origin of replication was defined for plasmids p353-2 and p8014-2 based on DNA sequence analysis and on its ability to convert single-stranded into double-stranded plasmid DNA. Plasmids pLPE323, pLPE350 and pLPC37 that are derived from the p353-2 or p8014-2 replicon are structurally and segregationally stable in L. pentosus MD353, L. plantarum ATCC 8014 and in Lactobacillus casei ATCC 393. The presence of Escherichia coli or lambda DNA fragments in vectors derived from p353-2 or p8014-2 does not affect the structural stability but results in segregational instability of the vectors. The instability increases with increasing size of the inserted DNA fragment. Since vectors based on these replicons can be efficiently propagated in a wide variety of Lactobacillus species, they are highly suitable for cloning and expression of foreign DNA in Lactobacillus, provided that selective pressure is applied.

Mutations affecting translational coupling between the rep genes of an IncB miniplasmid

The nature of translational coupling between repB and repA, the overlapping rep genes of the IncB plasmid pMU720, was examined. Mutations in the start codon of the promoter proximal gene, repB, reduced the efficiency of translation of both rep genes. Moreover, there was no independent initiation of repA translation in the absence of repB translation. The position of the repB stop codon was crucial for the efficient expression of repA, with the wild-type positioning being optimal. Translational coupling was found to be totally dependent on the formation of a pseudoknot structure. A model which invokes formation of a pseudoknot to facilitate initiation of repA is proposed.

Copy number control of the streptococcal plasmid pIP501 occurs at three levels

Transcriptional analysis of the replication region of plasmid pIP501 has revealed three active promoters. The repR gene which is essential for pIP501 replication was transcribed from promoter pII. A small antisense RNA (136 nt, RNAIII) generated from promoter pIII was complementary to the leader region of the repR mRNA. Introduction of either point mutations or deletions into promoter pIII or RNAIII resulted in a 5-20fold increased plasmid copy number suggesting a negative regulatory function for RNAIII. The copR gene, the complete DNA and amino acid sequence of which is reported, was dispensable for pIP501 replication. However, deletion of the copR promoter pI and/or the copR coding sequence led to a 10-20fold increase in plasmid copy number. This effect was also observed when a -1 frameshift mutation was introduced into the CopR coding region. Mutations in copR and pIII/RNAIII were not additive. It is, therefore, proposed that both components act at the same level of copy number control most likely in a sequential way. A second level of copy number control was found to involve an inverted repeat structure upstream of and overlapping with promoter pII. Destruction of this repeat sequence by deletion caused an increase in copy number 2-3fold higher than that observed for either RNAIII or copR mutations. A working model is proposed how different components of pIP501 interact to regulate its copy number.

The copy number of plasmid pLS1 is regulated by two trans-acting plasmid products: the antisense RNA II and the repressor protein, RepA

The promiscuous plasmid pLS1 encodes two transacting elements that regulate its copy number: protein RepA and antisense RNA II. In vitro transcription showed that RNAs for both repressors are synthesized from two promoters, PAB and PII. From PAB, genes encoding RepA (transcriptional repressor) and RepB (initiator of replication) are cotranscribed, the target of RepA being located within PAB. Mutants in repA or in PAB are still sensitive to RepA. However, cloning of the repA gene in a compatible replicon did not result in incompatibility towards pLS1. From PII, the 50-nucleotide RNA II is synthesized. The main incompatibility determinant towards pLS1 corresponds to the coding sequence for RNA II. The RNA II target could be reduced to 21 nucleotides, including the RepB initiation of translation signals. We propose that plasmids of the pLS1 family (pE194, pADB201, and pLB4) share functional and structural characteristics for the regulation of their copy numbers.

Regulation of the cadA cadmium resistance determinant of Staphylococcus aureus plasmid pI258

Regulation of the cadA cadmium and zinc resistance determinant of Staphylococcus aureus plasmid pI258 was demonstrated by using gene fusions and direct measurements of transcription. In growth experiments, cells harboring the intact cadA operon were induced with different cations and challenged by an inhibitory concentration of ZnCl2, a substrate of the CadA resistance system. Uninduced cells did not grow for 8 h after Zn2+ addition, whereas induced cells grew in the presence Zn2+. Cd2+ was a strong inducer, and Bi3+ and Pb2+ also induced well; Co2+ and Zn2+ were weak inducers. A translational beta-lactamase fusion to the cadA gene showed the same induction specificity as that seen with growth experiments with the intact cadA operon. A short beta-lactamase transcriptional fusion to the cadC gene also showed the same pattern of induction, establishing that the cadC gene was not involved in regulation. In Northern (RNA) blot hybridization experiments, a cadmium-inducible, 2.6-kb, operon-length transcript was detected. Primer extension experiments determined that Cd(2+)-inducible transcription of the cadA operon begins at nucleotides 676 and 677 of the published sequence (G. Nucifora, L. Chu, T. K. Misra, and S. Silver, Proc. Natl. Acad. Sci. USA 86: 3544-3548, 1989).

Regulation of replication of plasmid R1: an analysis of the intergenic region between copA and repA

The synthesis of the rate-limiting RepA replication initiator protein of plasmid R1 is negatively controlled by an antisense RNA, CopA. The regulation is posttranscriptional and involves an inhibitory effect on RepA translation mediated by the binding of CopA to its target (CopT) in the leader region of the RepA mRNA. The evolutionary conservation of the intergenic region between the copA gene and the repA reading frame among plasmids related to R1 may be indicative of an important function in this regulation. One possibility is that sequences/structures in this region might be required for the presumed distal effect of CopAQCopT binding. We have performed a mutational analysis of this region, starting with a mutant repA-lacZ fusion plasmid that shows decreased RepA-LacZ synthesis compared to a wild-type construct, and have identified five compensatory mutations that increase repA-lacZ expression. Two of these were single base-pair substitutions in the copA promoter leading to a decrease in CopA transcription. The other three mutations increased RepA synthesis in the presence as well as in the absence of functional CopA. Reconstructed plasmids carrying these mutations--in conjunction with the original down-mutation or in an otherwise wild-type background--show the expected increase in copy number. The effect of two of these mutations is consistent with the destabilization of a putative secondary structure which may be responsible for the normally low translation rate of the RepA reading frame. The implications of the types of mutations found in this study, as well as the absence of other classes of mutations, are discussed in terms of alternative possible models of CopA-mediated inhibition of RepA synthesis.

The Staphylococcus aureus mutation pcrA3 leads to the accumulation of pT181 replication initiation complexes

In Staphylococcus aureus cells carrying the pcrA3 chromosomal mutation, plasmid pT181 and its derivatives were maintained at a reduced copy number. A significant proportion of their DNA migrated during agarose gel electrophoresis as nicked DNA. The results obtained in the characterization of this plasmid DNA species show that it represents replication initiation complexes. Such complexes could not be detected in a wild-type host. The replication initiation complexes present in pcrA3 cells could resume replication after a lag. It was concluded from these results that the pcrA3 host mutation affected a step in plasmid pT181 replication immediately following the formation of the replication initiation complex, and that in pcrA3 this step became rate-limiting for plasmid pT181 replication.

The Staphylococcus aureus chromosomal gene plaC, identified by mutations amplifying plasmid pT181, encodes a sigma factor

The Staphylococcus aureus chromosomal gene plaC, identified by mutations such as plaC1 that lead to the amplification of plasmid pT181, has been cloned and sequenced. The plaC gene encodes a protein with high similarity (79% identity) with the vegetative sigma factor of Bacillus subtilis, sigA, suggesting that it acts as an RNA polymerase sigma factor in S.aureus. The plaC1 mutation was found to be a C to T transition leading to a proline to serine substitution at amino acid residue 209 of the protein. In other sigma factors this region of the protein is involved in specific recognition of the -10 promoter sequence. The change in sigma factor activity due to this mutation is characterized by its strict specificity for a limited number of promoters and the rather high amplitude of the effect.

Characterization and comparative sequence analysis of replication origins from three large Bacillus thuringiensis plasmids

The replication origins of three large Bacillus thuringiensis plasmids, derived from B. thuringiensis HD263 subsp. kurstaki, have been cloned in Escherichia coli and sequenced. The replication origins, designated ori 43, ori 44, and ori 60, were isolated from plasmids of 43, 44, and 60 MDa, respectively. Each cloned replication origin exhibits incompatibility with the resident B. thuringiensis plasmid from which it was derived. Recombinant plasmids containing the three replication origins varied in their ability to transform strains of B. thuringiensis, Bacillus megaterium, and Bacillus subtilis. Analysis of the derived nucleotide and amino acid sequences indicates that the replication origins are nonhomologous, implying independent derivations. No significant homology was found to published sequences of replication origins derived from the single-stranded DNA plasmids of gram-positive bacteria, and shuttle vectors containing the three replication origins do not appear to generate single-stranded DNA intermediates in B. thuringiensis. The replication origin regions of the large plasmids are each characterized by a single open reading frame whose product is essential for replication in B. thuringiensis. The putative replication protein of ori 60 exhibits partial homology to the RepA protein of the Bacillus stearothermophilus plasmid pTB19. The putative replication protein of ori 43 exhibits weak but extensive homology to the replication proteins of several streptococcal plasmids, including the open reading frame E replication protein of the conjugative plasmid pAM beta 1. The nucleotide sequence of ori 44 and the amino acid sequence of its putative replication protein appear to be nonhomologous to other published replication origin sequences.

Control of ColE1 plasmid replication by antisense RNA

One of the two major classes of regulatory strategies that control plasmid copy number involves recognition via base pairing between two plasmid-encoded complementary RNAs. The detailed analysis of this control circuitry has revealed some features of regulatory mechanisms based on RNA-RNA interaction that distinguish them from those based on protein-nucleic acid interaction. These features provide a framework with which to understand other regulatory mechanisms based on RNA-RNA interaction, and will aid in the design of efficient artificial antisense RNA systems.

Genetic analysis of a lactococcal plasmid replicon

The sequence and genetic organization was determined of the 2508 bp lactococcal portion of pFX2, which was derived from a cryptic Lactococcus lactis subsp. lactis plasmid and used as the basis for construction of a series of lactococcal vectors. A lactococcal plasmid plus origin and two replication protein-coding regions (repA and repB) were located. RepA has a helix-turn-helix motif, a geometry typical of DNA-binding proteins. RepB shows a high degree of homology to the plasmid replication initiation proteins from other gram-positive bacteria and Mycoplasma. The transcribed inverted repeat sequence between repA and repB could form an attenuator to regulate pFX2 replication. Up-stream of the ori site, and in a region which was non-essential for replication, a 215 bp sequence identical to the staphylococcal plasmid pE194 and carrying the RSA site was identified. The genetic organization of this lactococcal plasmid replicon shares significant similarity with pE194 group plasmids.

The effect of plasmid copy number mutations on pT181 replication initiator protein expression

Previous studies have shown that plasmid pT181 controls its replication by countertranscript-mediated regulation of the rate of synthesis of the pT181 initiator, RepC. In this study, the relation has been studied between plasmid copy number and RepC synthesis for a series of pT181 copy number mutants. For each mutant plasmid, the repC coding sequence along with its 5' regulatory region was translationally fused to the beta-lactamase structural gene on a vector plasmid unrelated to pT181. By means of these constructs, the effect of regulatory mutations on the initiator synthesis could be measured at constant copy number. With one exception, the mutant control regions showed elevated beta-lactamase activity in comparison to the wild-type. However, the relative increase was not very well correlated with the copy number of the corresponding mutant plasmid. The possibility is considered that factors such as DNA secondary structure may have important ancillary effects on the regulation mechanism.

Stable RNA secondary structure in a retroviral vector insert terminates reverse transcriptase elongation in vitro but not in cultured cells

We wished to test whether an RNA signal that causes termination of elongation by reverse transcriptase in vitro would affect retroviral vector function. A synthetic oligonucleotide containing a sequence capable of forming a very stable RNA secondary structure was subcloned into the retrovirus vector N2. The integration of this sequence into N2 causes termination of elongation by reverse transcriptase in vitro at the precise positions previously reported in a different sequence context. However, no premature termination of DNA synthesis was observed in the unintegrated DNA of vector transduced cells. Likewise, there was no deleterious effect of the sequence insert on vector titer. These results indicate that termination signals defined in in vitro systems cannot be used as predictors of in vivo function and suggest that viral proteins in addition to reverse transcriptase play an important role in transcript initiation and elongation.

Lactobacillus hilgardii plasmid pLAB1000 consists of two functional cassettes commonly found in other gram-positive organisms

A Lactobacillus hilgardii plasmid, pLAB1000, was studied to understand the organization of autonomous replicons from lactobacilli. Two cassettes could be identified. First, the replication region consisted of a sequence coding for a replication protein (Rep) and its corresponding target site, similar to those from plasmids pUB110, pC194 (Staphylococcus aureus), pFTB14, pBAA1 (Bacillus sp.), and pLP1 (Lactobacillus sp.). Sequence analysis indicated the possible synthesis of an antisense RNA that might regulate Rep production. The results also suggested that pLAB1000 replicates via a single-stranded DNA intermediate, and a putative lagging-strand initiation site was found that had similarities to those of alpha 3, St-1, and G4 isometric bacteriophages. The second cassette of pLAB1000 consisted of a sequence coding for a putative mobilization protein (Mob) and its corresponding RSA site. This cassette was similar to those found in pT181, pUB110, pE194 (S. aureus), and pG12 (Bacillus sp.), and it was found to be conserved among different Lactobacillus plasmid replicons. The origin and evolution of these functional cassettes are also discussed.