Characterization of the Bacillus subtilis tryptophan promoter region

New Bacillus subtilis vector, pSSβ, as genetic tool for site-specific integration and excision of cloned DNA, and prophage elimination

Site-specific recombination (SSR) systems are employed in many genetic mobile elements, including temperate phages, for their integration and excision. Recently, they have also been used as tools for applications in fields ranging from basic to synthetic biology. SPβ is a temperate phage of the Siphoviridae family found in the laboratory standard Bacillus subtilis strain 168. SPβ encodes a serine-type recombinase, SprA, and recombination directionality factor (RDF), SprB. SprA catalyzes recombination between the attachment site of the phage, attP, and that of the host, attB, to integrate phage genome into the attB site of the host genome and generate attL and attR at both ends of the prophage genome. SprB works in conjunction with SprA and switches from attB/attP to attL/R recombination, which leads to excision of the prophage. In the present study, we took advantage of this highly efficient recombination system to develop a site-specific integration and excision plasmid vector, named pSSβ. It was constructed using pUC plasmid and the SSR system components, attP, sprA and sprB of SPβ. pSSβ was integrated into the attB site with a significantly high efficiency, and the resulting pSSβ integrated strain also easily eliminated pSSβ itself from the host genome by the induction of SprB expression with xylose. This report presents two applications using pSSβ that are particularly suitable for gene complementation experiments and for a curing system of SPβ prophage, that may serve as a model system for the removal of prophages in other bacteria.

Identification and characterization of mutations conferring resistance to D-amino acids in Bacillus subtilis

Bacteria produce d-amino acids for incorporation into the peptidoglycan and certain nonribosomally produced peptides. However, D-amino acids are toxic if mischarged on tRNAs or misincorporated into protein. Common strains of the Gram-positive bacterium Bacillus subtilis are particularly sensitive to the growth-inhibitory effects of D-tyrosine due to the absence of D-aminoacyl-tRNA deacylase, an enzyme that prevents misincorporation of D-tyrosine and other D-amino acids into nascent proteins. We isolated spontaneous mutants of B. subtilis that survive in the presence of a mixture of D-leucine, D-methionine, D-tryptophan, and D-tyrosine. Whole-genome sequencing revealed that these strains harbored mutations affecting tRNA(Tyr) charging. Three of the most potent mutations enhanced the expression of the gene (tyrS) for tyrosyl-tRNA synthetase. In particular, resistance was conferred by mutations that destabilized the terminator hairpin of the tyrS riboswitch, as well as by a mutation that transformed a tRNA(Phe) into a tyrS riboswitch ligand. The most potent mutation, a substitution near the tyrosine recognition site of tyrosyl-tRNA synthetase, improved enzyme stereoselectivity. We conclude that these mutations promote the proper charging of tRNA(Tyr), thus facilitating the exclusion of D-tyrosine from protein biosynthesis in cells that lack D-aminoacyl-tRNA deacylase.

IMPORTANCE

Proteins are composed of L-amino acids. Mischarging of tRNAs with D-amino acids or the misincorporation of D-amino acids into proteins causes toxicity. This work reports on mutations that confer resistance to D-amino acids and their mechanisms of action.

The rate of TRAP binding to RNA is crucial for transcription attenuation control of the B. subtilis trp operon

The trp RNA-binding attenuation protein (TRAP) regulates expression of the tryptophan biosynthetic and transport genes in Bacillus subtilis in response to changes in the levels of intracellular tryptophan. Transcription of the trpEDCFBA operon is controlled by an attenuation mechanism involving two overlapping RNA secondary structures in the 5' leader region of the trp transcript; TRAP binding promotes formation of a transcription terminator structure that halts transcription prior to the structural genes. TRAP consists of 11 identical subunits and is activated to bind RNA by binding up to 11 molecules of L-tryptophan. The TRAP binding site in the leader region of the trp operon mRNA consists of 11 (G/U)AG repeats. We examined the importance of the rate of TRAP binding to RNA for the transcription attenuation mechanism. We compared the properties of two types of TRAP 11-mers: homo-11-mers composed of 11 wild-type subunits, and hetero-11-mers with only one wild-type subunit and ten mutant subunits defective in binding either RNA or tryptophan. The hetero-11-mers bound RNA with only slightly diminished equilibrium binding affinity but with slower on-rates as compared to WT TRAP. The hetero-11-mers showed significantly decreased ability to induce transcription termination in the trp leader region when examined using an in vitro attenuation system. Together these results indicate that the rate of TRAP binding to RNA is a crucial factor in TRAP's ability to control attenuation.

Transcriptional termination control of a novel ABC transporter gene involved in antibiotic resistance in Bacillus subtilis

In members of one of the subfamilies of the bacterial ATP binding cassette (ABC) transporters, the two nucleotide binding domains are fused as a single peptide and the proteins have no membrane-spanning domain partners. Most of the ABC efflux transporters of this subfamily have been characterized in actinomycetes, producing macrolide, lincosamide, and streptogramin antibiotics. Among 40 ABC efflux transporters of Bacillus subtilis, five proteins belong to this subfamily. None of these proteins has been functionally characterized. We examined macrolide, lincosamide, and streptogramin antibiotic resistance in insertional disruptants of the genes that encode these proteins. It was found that only a disruptant of vmlR (formerly named expZ) showed hypersensitivity to virginiamycin M and lincomycin. Expression of the vmlR gene was induced by the addition of these antibiotics in growth medium. Primer extension analysis revealed that transcription of the vmlR gene initiates at an adenosine residue located 225 bp upstream of the initiation codon. From the analysis of the vmlR and lacZ fusion genes, a 52-bp deletion from +159 to +211 resulted in constitutive expression of the vmlR gene. In this region, a typical rho-independent transcriptional terminator was found. It was suggested that the majority of transcription ends at this termination signal in the absence of antibiotics, whereas under induced conditions, RNA polymerase reads through the terminator, and transcription continues to the downstream vmlR coding region, resulting in an increase in vmlR expression. No stabilization of vmlR mRNA occurred under the induced conditions.

Bacillus subtilis YdiH is a direct negative regulator of the cydABCD operon

During aerobic respiration, Bacillus subtilis utilizes three terminal oxidases, cytochromes aa3, caa3, and bd. Cytochrome bd is encoded by the cydABCD operon. We report here the first identification of a regulator for the cydABCD operon, YdiH. While working with DeltaresDE mutant strains, we identified colonies which contained suppressor mutations (cmp) which bypassed the requirement for ResD for all phenotypes not associated with cytochrome aa3 or caa3. Mapping identified a class of Tn10 insertions which were close to the cmp locus (Tn10-2) and a second class (Tn10-1) which was inserted in cydD, a gene which appears to be essential to the cmp phenotype. Sequencing of the cmp loci from four independent DeltaresDE cmp isolates yielded four loss-of-function alleles of ydiH, a gene encoding a protein with homology to AT-rich DNA-binding proteins. Additionally, we determined that cytochrome bd was aberrantly expressed in the DeltaresDE cmp background. Together these data led to the hypothesis that YdiH serves as a negative regulator of cydABCD expression, a hypothesis supported by both gel-shift and DNase I footprinting analyses. YdiH protected the cydA promoter region at three 22-bp repeats located in the long 5' untranslated region (193 bp). Induction of the cydABCD operon in a DeltaresDE background showed that expression of the terminal oxidase bd was responsible for the bypass phenotype observed in a DeltaresDE cmp strain, indicating that cytochrome bd expression complemented the loss of cytochromes aa3 and caa3 in the DeltaresDE strain.

New shuttle vectors for ectopic insertion of genes into Bacillus subtilis

We have constructed shuttle vectors for integration of genes via double homologous recombination into three ectopic sites on the chromosome of Bacillus subtilis. The sites of integration are the pyrD, gltA, and sacA genes located at 139 degrees, 172 degrees, and 333 degrees, respectively, on the chromosome. Integration of the vectors into the target genes leads to antibiotic resistance as well as different metabolic phenotypes. B. subtilis strains with integrations of the empty vectors were able to sporulate at rates comparable to wild type cells. Similar levels of expression were obtained from constitutive lacZ fusions integrated at the different sites.

Regulation of the tryptophan biosynthetic genes in Bacillus halodurans: common elements but different strategies than those used by Bacillus subtilis

In Bacillus subtilis, an RNA binding protein called TRAP regulates both transcription and translation of the tryptophan biosynthetic genes. Bacillus halodurans is an alkaliphilic Bacillus species that grows at high pHs. Previous studies of this bacterium have focused on mechanisms of adaptation for growth in alkaline environments. We have characterized the regulation of the tryptophan biosynthetic genes in B. halodurans and compared it to that in B. subtilis. B. halodurans encodes a TRAP protein with 71% sequence identity to the B. subtilis protein. Expression of anthranilate synthetase, the first enzyme in the pathway to tryptophan, is regulated significantly less in B. halodurans than in B. subtilis. Examination of the control of the B. halodurans trpEDCFBA operon both in vivo and in vitro shows that only transcription is regulated, whereas in B. subtilis both transcription of the operon and translation of trpE are controlled. The attenuation mechanism that controls transcription in B. halodurans is similar to that in B. subtilis, but there are some differences in the predicted RNA secondary structures in the B. halodurans trp leader region, including the presence of a potential anti-antiterminator structure. Translation of trpG, which is within the folate operon in both bacilli, is regulated similarly in the two species.

Development of a two-part transcription probe to determine the completeness of temporal and spatial compartmentalization of gene expression during bacterial development

We have developed a two-part test, using the Bacillus subtilis sacB/SacY transcription antitermination system, to evaluate the completeness of temporal and spatial compartmentalization of gene expression during bacterial cell development. Transcription of sacY(1-55) (encoding a constitutively active form of the antiterminator, SacY) is directed by one promoter, whereas transcription of sacB'-'lacZ (the target of SacY action) is directed by the same or another promoter. To obtain beta-galactosidase activity, SacY(1-55) needs to be present when sacB'-'lacZ is being transcribed. We tested the system by analyzing the spatial compartmentalization of the activities of RNA polymerase final sigma factors, which are tightly regulated during sporulation of B. subtilis: final sigma(F) and then final sigma(G) in the prespore, final sigma(E) and then final sigma(K) in the mother cell. We have confirmed that the activities of final sigma(F) and final sigma(E) are spatially compartmentalized. We have demonstrated that there is also sharp temporal compartmentalization, with little or no overlap in the activities of final sigma(F) and final sigma(G) or of final sigma(E) and final sigma(K). In contrast, we found no compartmentalization of the activity of the main vegetative factor, final sigma(A), which continued to be active alongside all of the sporulation-specific final sigma factors. We also found no temporal compartmentalization of expression of loci that are activated during the development of competent cells of B. subtilis, a developmental program distinct from spore formation. A possible mechanism to explain the temporal compartmentalization of final sigma(F) and final sigma(G) activities is that the anti-sigma factor SpoIIAB transfers from final sigma(G) to final sigma(F).

Regulatory features of the trp operon and the crystal structure of the trp RNA-binding attenuation protein from Bacillus stearothermophilus

Characterization of both the cis and trans -acting regulatory elements indicates that the Bacillus stearothermophilustrp operon is regulated by an attenuation mechanism similar to that which controls the trp operon in Bacillus subtilis. Secondary structure predictions indicate that the leader region of the trp mRNA is capable of folding into terminator and anti- terminator RNA structures. B. stearothermophilus also encodes an RNA-binding protein with 77% sequence identity with the RNA-binding protein (TRAP) that regulates attenuation in B. subtilis. The X-ray structure of this protein has been determined in complex with L-tryptophan at 2.5 A resolution. Like the B. subtilis protein, B. stearothermophilus TRAP has 11 subunits arranged in a ring-like structure. The central cavities in these two structures have different sizes and opposite charge distributions, and packing within the B. stearothermophilus TRAP crystal form does not generate the head-to-head dimers seen in the B. subtilis protein, suggesting that neither of these properties is functionally important. However, the mode of L-tryptophan binding and the proposed RNA binding surfaces are similar, indicating that both proteins are activated by l -tryptophan and bind RNA in essentially the same way. As expected, the TRAP:RNA complex from B. stearothermophilus is significantly more thermostable than that from B. subtilis, with optimal binding occurring at 70 degrees C.

Compilation and analysis of Bacillus subtilis sigma A-dependent promoter sequences: evidence for extended contact between RNA polymerase and upstream promoter DNA

Sequence analysis of 236 promoters recognized by the Bacillus subtilis sigma A-RNA polymerase reveals an extended promoter structure. The most highly conserved bases include the -35 and -10 hexanucleotide core elements and a TG dinucleotide at position -15, -14. In addition, several weakly conserved A and T residues are present upstream of the -35 region. Analysis of dinucleotide composition reveals A2- and T2-rich sequences in the upstream promoter region (-36 to -70) which are phased with the DNA helix: An tracts are common near -43, -54 and -65; Tn tracts predominate at the intervening positions. When compared with larger regions of the genome, upstream promoter regions have an excess of An and Tn sequences for n > 4. These data indicate that an RNA polymerase binding site affects DNA sequence as far upstream as -70. This sequence conservation is discussed in light of recent evidence that the alpha subunits of the polymerase core bind DNA and that the promoter may wrap around RNA polymerase.

The mtrB gene of Bacillus pumilus encodes a protein with sequence and functional homology to the trp RNA-binding attenuation protein (TRAP) of Bacillus subtilis

The mtrB gene from Bacillus pumilus encodes a 76-amino-acid polypeptide with 77% identity to the trp RNA-binding attenuation protein (TRAP) from Bacillus subtilis. B. pumilus TRAP binds trp leader RNA from either B. subtilis or B. pumilus in a tryptophan-dependent manner. Altering threonine 52 to alanine eliminated RNA-binding activity of B. pumilus TRAP.

A small (2.4 Mb) Bacillus cereus chromosome corresponds to a conserved region of a larger (5.3 Mb) Bacillus cereus chromosome

We have determined the sizes of the chromosomes of six Bacillus cereus strains (range 2.4-4.3 Mb) and constructed a physical map of the smallest B. cereus chromosome (2.4 Mb). This map was compared to those of the chromosomes of four B. cereus strains and one B. thuringiensis strain previously determined to be 5.4-6.3 Mb. Of more than 50 probes, 30 were localized to the same half of the larger B. cereus and B. thuringiensis chromosomes. All 30 were also present on the small chromosome. Twenty of the probes present on the other half of the larger chromosomes were either present on extrachromosomal DNA, or absent from the B. cereus strain carrying the small chromosome. We propose that the genome of B. cereus/B. thuringiensis has one constant part and another less stable part which is more easily mobilized into other genetic elements. This part of the genome is localized to one region of the chromosome and may be subject to deletions or more frequent relocations between the chromosome and episomal elements of varying sizes up to the order of megabases.

Regulation of the Bacillus subtilis trp operon by an RNA-binding protein

The Bacillus subtilis tryptophan (trpEDCFBA) operon is regulated by transcription attenuation. Transcription is controlled by two alternative RNA secondary structures, which form in the leader transcript. In the presence of L-tryptophan, a transcription terminator forms and the operon is not expressed, whereas in the absence of tryptophan, an antiterminator structure forms allowing transcription of the operon. The mechanism of selection between these alternative structures involves a trans-acting RNA-binding regulatory protein. This protein is the product of the mtrB gene and is called TRAP for trp attenuation protein. TRAP has been shown to bind specifically to trp leader RNA, and to cause transcription of the trp operon to terminate in the leader region. The model for regulation suggests that in the presence of tryptophan, TRAP binds to the leader RNA and induces formation of the transcription terminator structure, whereas in the absence of tryptophan, the protein does not bind and the antiterminator is formed.

Reconstitution of Bacillus subtilis trp attenuation in vitro with TRAP, the trp RNA-binding attenuation protein

We have reconstituted Bacillus subtilis trp attenuation in vitro. Purification of the mtrB gene product (TRAP) to near homogeneity allowed us to demonstrate that addition of this protein plus L-tryptophan to template, RNA polymerase, and nucleoside triphosphates caused transcription termination in the trpEDCFBA leader region. TRAP acts by binding to the nascent transcript and preventing formation of an RNA antiterminator structure, thereby allowing terminator formation and transcription termination. Oligonucleotides complementary to segments of the antiterminator were used to demonstrate that formation of this RNA hairpin was responsible for transcription read-through. TRAP was found to be a 60-kDa multimeric protein composed of identical 6- to 8-kDa subunits, and its elution profile on a chromatographic column did not change in the presence of tryptophan.

MtrB from Bacillus subtilis binds specifically to trp leader RNA in a tryptophan-dependent manner

MtrB regulates transcription attenuation of the Bacillus subtilis trp operon. We have shown that MtrB, either from B. subtilis or overexpressed in Escherichia coli, binds specifically to RNA from the leader region of the trp operon by a gel mobility-shift assay. This binding is tryptophan dependent. MtrB binds to a transcript terminated at the trp attenuator (-2 to +138) or a read-through transcript (-2 to +318). MtrB does not bind antisense trp leader RNA or single-stranded trp leader DNA. These results support the model in which attenuation is controlled by tryptophan-activated MtrB influencing the secondary structure of the leader region transcript to form a terminator structure.

Transcriptional induction of Streptomyces cacaoi beta-lactamase by a beta-lactam compound

The soil bacterium Streptomyces cacaoi produces an extracellular beta-lactamase. The beta-lactamase expression could be induced by the beta-lactam compound 6-amino penicillinoic acid (6-APA). In liquid cultures, a 50-fold increase in beta-lactamase expression was observed within the first three hours after addition of 6-APA. Using the cloned beta-lactamase gene as a probe, it was shown that this increase was mediated at the level of transcriptional initiation. The start point of the induced beta-lactamase transcript was determined, and the nucleotide sequence of the promoter region was analysed. No noticeable homology was found to control regions of inducible beta-lactamase genes of other bacteria. A striking feature was the presence of six direct repeats (ten base pairs each) upstream of the promoter region. Thus, an example of an inducible regulatory gene system in this Gram-positive microorganism is presented. Also, the primary structure of the beta-lactamase was deduced, showing a high degree of homology with class A beta-lactamases.

In vivo selected promoter and ribosome binding site up-mutations: demonstration that the Escherichia coli bla promoter and a Shine-Dalgarno region with low complementarity to the 16 S ribosomal RNA function in Bacillus subtilis

We have constructed a plasmid, pQS1, in which a mouse dihydrofolate reductase (5,6,7,8-tetrahydrofolate:NADP:oxidoreductase; EC 1.5.1.3; DHFR) cDNA is inserted in the unique PstI site of a gram-positive/gram-negative shuttle vector derived from pBR322. The cDNA is expressed under the control of the bla promoter, which, like most gram-negative bacterial genes, is considered not to be expressed in Bacillus subtilis, and its coding sequence is translated from a polycistronic message. We have selected in vivo and studied, in Escherichia coli and B. subtilis, expression mutants with promoter and ribosome binding site sequence mutations. One promoter mutation changes the third nucleotide of the -35 region from a C to a G. As expected, this substitution results in increased transcriptional activity in E. coli. In B. subtilis, this mutation induces the accumulation not only of a low but significant amount of dhfr mRNA but also of DHFR, demonstrating that binding strengths with a free energy as low as -9.4 kcal/mol are sufficient to promote ribosome binding in B. subtilis. The association of the promoter mutation (C-G) with a mutation which creates a strong B. subtilis ribosome binding site (-21 kcal/mol) results in the accumulation of a large amount of dhfr mRNA. This demonstrates the importance of having an efficient ribosome binding site in the evaluation of promoter function: for example, with this strong ribosome binding site we can show that the wild-type bla promoter is recognized by the B. subtilis transcription machinery.

cis-acting sites in the transcript of the Bacillus subtilis trp operon regulate expression of the operon

Transcription of the trp operon of Bacillus subtilis is regulated by attenuation. A trpE'-'lacZ gene fusion preceded by the wild-type trp promoter-leader region was used to analyze regulation. Overproduction of the trp leader transcript in trans from a multicopy plasmid caused constitutive expression of the chromosomal trpE'-'lacZ fusion, presumably by titrating a negative regulatory factor encoded by the mtr locus. Subsegments of the trp leader region cloned onto the multicopy plasmid were examined for their abilities to elevate beta-galactosidase activity. An RNA segment spanning the portion of the leader transcript that forms the promoter-proximal strand of the proposed antiterminator structure was most active in this trans test. The data suggest that the mtr gene product, when activated by tryptophan, binds to this RNA segment and prevents formation of the antiterminator. In this manner, the trans-acting factor promotes formation of the RNA structure that causes transcription termination. Secondary-structure predictions for the leader segment of the trp operon transcript suggest that if the mtr factor bound this RNA segment in a nonterminated transcript, the ribosome-binding site for the first structural gene, trpE, could be sequestered in a stable RNA structure. We tested this possibility by comparing transcriptional and translational fusions containing the initial segments of the trp operon. Our findings suggest that the mtr product causes both transcription attenuation and inhibition of translation of trpE mRNA. Inhibition of translation initiation would reduce ribosome density on trpE mRNA, perhaps making it more labile. Consistent with this interpretation, the addition of tryptophan to mtr+ cultures increased the rate of trpE'-'lacZ mRNA decay.

Modulation of Bacillus subtilis levansucrase gene expression by sucrose and regulation of the steady-state mRNA level by sacU and sacQ genes

In Bacillus subtilis, the extracellular enzyme levansurcrase is synthesized in the presence of sucrose. A termination structure between the transcription start site and the structural gene was the apparent site for regulation by sucrose of transcription into the structural gene. Sequence analysis of the sacB leader region from two strains constitutive for levansucrase synthesis showed a single base change in the stem of this termination structure. This single base change also led to the constitutive synthesis of a sacB'-'lacZ fusion, whereas the wild-type sacB'-'lacZ fusion was induced by the addition of sucrose. S1 nuclease mapping of sacB transcripts with probes labeled either within the termination structure or 3' to the termination structure showed that sucrose did not increase the number of transcripts extending into the termination structure; however, sucrose did increase the number of transcripts extending past the termination structure. Two pleiotropic mutations which affect the expression of levansucrase, sacQ36 hyperproducing [sacQ36(Hy)] and sacU32(Hy), were separately introduced into the strain carrying the sacB'-'lacZ fusion. These mutations each increased the expression levels of the sacB'-'lacZ fusion. S1 mapping showed increased levels of transcript initiating at the sacB promoter in strains with the sacQ36(Hy) and sacU32(Hy) mutations. This increased transcription appeared to be independent of the sucrose-regulated transcription termination, suggesting the existence of at least two different mechanisms for the regulation of sacB expression.

Regulatory elements common to the Bacillus pumilus and Bacillus subtilis trp operons

The trp operon regulatory region of Bacillus pumilus was cloned and sequenced. The cloned B. pumilus trp promoter-leader region functioned in Bacillus subtilis to express the adjacent leukocyte interferon A gene on a multicopy transcriptional fusion plasmid, pBpIFI. In strains carrying this plasmid, anthranilate synthetase levels were elevated, possible due to titration of a B. subtilis trp regulatory factor by multiple copies of the transcript of the plasmid-borne B. pumilus trp leader region. The B. pumilus trp promoter was recognized efficiently in vitro by B. subtilis sigma 43 RNA polymerase. Approximately 12% of the transcripts produced in vitro terminated in the leader region immediately following synthesis of a transcript structure resembling rho-independent terminators of enteric bacteria. An analogous terminator exists in the B. subtilis trp leader transcript. Nucleotide sequence comparison of the B. pumilus and B. subtilis trp leader regions revealed conservation of these and other sequences that could form transcript secondary structures postulated to regulate transcription termination in B. subtilis (H. Shimotsu, M.I. Kuroda, C. Yanofsky, and D.J. Henner, J. Bacteriol. 166:461-471, 1986). We propose that two elements implicated in B. subtilis trp operon regulation are conserved in the related organism B. pumilus: alternative transcription antiterminator and terminator structures in the leader transcript, and a trans-acting factor present in limiting amounts that is required for transcription termination in the leader region.

Isolation and characterization of a 2.2-kb operon preceding the alpha-amylase gene of Bacillus amyloliquefaciens

A DNA region of 2.8 X 10(3) base pairs (2.8 kb) upstream of the Bacillus amyloliquefaciens alpha-amylase gene has been isolated. This DNA gave rise to a 2.2-kb transcript. The 3' end of the transcript was mapped with S1 nuclease and shown to terminate 49 base pairs upstream of the -35 region of the alpha-amylase promoter. In B. subtilis minicells this 2.2-kb transcript coded for three different polypeptides, thus indicating a polycistronic operon-type structure. The location and the order of the polypeptides were established using DNA deletions. The joining of the 2.2-kb operon to the downstream alpha-amylase gene in the plasmid pUB110 did not have any significant effect on the level of expression of the alpha-amylase.

A transcription terminator in the 5' non-coding region of the tyrosyl tRNA synthetase gene from Bacillus stearothermophilus

The 5' non-coding region of the tyrosyl-tRNA synthetase gene (tyrS) of Bacillus stearothermophilus is 324 nucleotides long. It contains a premature terminator and a strong promoter: these were identified in vitro by RNA run-off experiments and in Escherichia coli by construction of specific mutants. The terminator consists of a stem and loop structure followed by the string of T residues characteristic of rho-independent termination. This is preceded by another stem and loop structure which may permit the formation of an anti-terminator. Neither the promoter nor the premature terminator appears to be regulated by the tyrosyl-tRNA synthetase in vitro or in E. coli.

Novel form of transcription attenuation regulates expression the Bacillus subtilis tryptophan operon

Transcription of the trp operon of Bacillus subtilis is regulated in response to the availability of tryptophan. The first structural gene of the operon is preceded by a 204-base-pair transcribed leader region that contains a segment with the features of a procaryotic termination site. Transcription of the leader region was analyzed in vivo and in vitro to determine whether this putative termination site was used to regulate operon expression. When RNA was isolated from wild-type cells grown in the presence of excess tryptophan, transcripts of the operon ended at the putative termination site. In contrast, RNA isolated from cells grown in the absence of tryptophan or from a mutant strain which is constitutive for trp operon expression contained trp transcripts that extended beyond the termination site into the structural genes. To assess termination quantitatively in vivo, a trpE-lacZ fusion was constructed in which the trp promoter and leader region controls hybrid beta-galactosidase formation. The effects on hybrid beta-galactosidase levels of point mutations and deletions introduced into this leader region were determined. The results obtained establish that transcription of the trp operon structural genes is regulated in the leader region. This regulation appears to be mediated by the formation of alternative secondary structures of the leader transcript. In vitro transcription studies with wild-type and mutant templates provided additional evidence that the identified alternative RNA secondary structures regulate transcription termination. We hypothesize that binding of a tryptophan-activated regulatory protein to a specific segment of the nascent leader transcript prevents formation of one of the alternative secondary structures, thereby directing RNA polymerase to terminate transcription.

Identification and nucleotide sequence of the promoter region of the Bacillus subtilis gluconate operon

The nucleotide sequence (742 bp) of the promoter region of the Bacillus subtilis gluconate (gnt) operon is presented. Nuclease Sl mapping revealed the start point of the transcription and suggested that the expression of this operon is probably regulated at the transcriptional level. The sequences of the -35 and -10 regions suggested that RNA polymerase possessing sigma-43 may recognize this structure. The 223 bp fragment containing 100 bp upstream from the transcription start site actually exhibited a promoter activity when cloned in a promoter probe vector of pPL603B. This promoter activity was highly derepressed and although still under catabolite repression. The fragment on a high copy plasmid could titrate a regulator of the gnt operon so that the expression of the operon on the host chromosome also became derepressed.

Construction and properties of Tn917-lac, a transposon derivative that mediates transcriptional gene fusions in Bacillus subtilis

A derivative of Tn917 was constructed, referred to as Tn917-lac, which is capable of generating fusions that connect the transcripts of Bacillus subtilis chromosomal genes to the coding sequence of the lacZ gene of Escherichia coli. Two independent insertions of Tn917-lac into the gltA gene and one insertion into the trpE gene (in the trpEDCFBA operon) of B. subtilis were studied in detail, and the results confirmed that Tn917-lac-mediated transcriptional fusions produce levels of beta-galactosidase that reflect accurately the regulated expression of interrupted genes. To facilitate these studies, a procedure was developed that permits the analysis of Tn917-lac-mediated fusions in partial diploids where insertional mutations are complemented by an intact copy of the interrupted genes. Tn917 is known to function efficiently in bacteria representing three quite different Gram-positive genera (Streptococcus, Bacillus, and Staphylococcus) and is known to display a relatively high degree of randomness in its insertions into bacterial genomes, making it likely that Tn917-lac will be useful for the identification and study of many kinds of regulated genes in a wide range of Gram-positive species.

[Genetic analysis of sacR, a cis-regulator of levan-saccharase synthesis of Bacillus subtilis]

The phenotypical and physical characterization of point mutations and deletions affecting sacR were analysed. The 2 X 28-bp palindromic region present within sacR was responsible for sacB inducibility by sucrose. The sacR mutations also affected sacB expression in Escherichia coli, suggesting that the palindromic region is a transcriptional terminator. It is difficult to apply classical bacterial models of negative or positive regulation to sacR function in B. subtilis. We propose that sacR works as a transcriptional attenuator. Our hypothesis is similar to that proposed for the regulation of the tryptophan operon in B. subtilis: the attenuation of sacB transcription could be modulated by a diffusible regulator whose activity is controlled by the inducer, sucrose. Furthermore, we have observed very strong sequence homologies between sacR and the region located upstream from the gene of another secreted enzyme from B. subtilis, beta-glucanase. We propose a preliminary discussion of this observation.