cis-acting sequences of Bacillus subtilis pyrG mRNA essential for regulation by antitermination

Temperature-Sensitive Salmonella enterica Serovar Enteritidis PT13a Expressing Essential Proteins of Psychrophilic Bacteria

Synthetic genes based on deduced amino acid sequences of the NAD-dependent DNA ligase (ligA) and CTP synthetase (pyrG) of psychrophilic bacteria were substituted for their native homologues in the genome of Salmonella enterica serovar Enteritidis phage type 13a (PT13a). The resulting strains were rendered temperature sensitive (TS) and did not revert to temperature resistance at a detectable level. At permissive temperatures, TS strains grew like the parental strain in broth medium and in macrophage-like cells, but their growth was slowed or stopped when they were shifted to a restrictive temperature. When injected into BALB/c mice at the base of the tail, representing a cool site of the body, the strains with restrictive temperatures of 37, 38.5, and 39°C persisted for less than 1 day, 4 to 7 days, and 20 to 28 days, respectively. The wild-type strain persisted at the site of inoculation for at least 28 days. The wild-type strain, but not the TS strains, was also found in spleen-plus-liver homogenates within 1 day of inoculation of the tail and was detectable in these organs for at least 28 days. Intramuscular vaccination of White Leghorn chickens with the PT13a strain carrying the psychrophilic pyrG gene provided some protection against colonization of the reproductive tract and induced an anti-S. enterica antibody response.

Human body temperature and new approaches to constructing temperature-sensitive bacterial vaccines

Many of the live human and animal vaccines that are currently in use are attenuated by virtue of their temperature-sensitive (TS) replication. These vaccines are able to function because they can take advantage of sites in mammalian bodies that are cooler than the core temperature, where TS vaccines fail to replicate. In this article, we discuss the distribution of temperature in the human body, and relate how the temperature differential can be exploited for designing and using TS vaccines. We also examine how one of the coolest organs of the body, the skin, contains antigen-processing cells that can be targeted to provoke the desired immune response from a TS vaccine. We describe traditional approaches to making TS vaccines, and highlight new information and technologies that are being used to create a new generation of engineered TS vaccines. We pay particular attention to the recently described technology of substituting essential genes from Arctic bacteria for their homologues in mammalian pathogens as a way of creating TS vaccines.

Regulation of pyrimidine biosynthetic gene expression in bacteria: repression without repressors

SUMMARY

DNA-binding repressor proteins that govern transcription initiation in response to end products generally regulate bacterial biosynthetic genes, but this is rarely true for the pyrimidine biosynthetic (pyr) genes. Instead, bacterial pyr gene regulation generally involves mechanisms that rely only on regulatory sequences embedded in the leader region of the operon, which cause premature transcription termination or translation inhibition in response to nucleotide signals. Studies with Escherichia coli and Bacillus subtilis pyr genes reveal a variety of regulatory mechanisms. Transcription attenuation via UTP-sensitive coupled transcription and translation regulates expression of the pyrBI and pyrE operons in enteric bacteria, whereas nucleotide effects on binding of the PyrR protein to pyr mRNA attenuation sites control pyr operon expression in most gram-positive bacteria. Nucleotide-sensitive reiterative transcription underlies regulation of other pyr genes. With the E. coli pyrBI, carAB, codBA, and upp-uraA operons, UTP-sensitive reiterative transcription within the initially transcribed region (ITR) leads to nonproductive transcription initiation. CTP-sensitive reiterative transcription in the pyrG ITRs of gram-positive bacteria, which involves the addition of G residues, results in the formation of an antiterminator RNA hairpin and suppression of transcription attenuation. Some mechanisms involve regulation of translation rather than transcription. Expression of the pyrC and pyrD operons of enteric bacteria is controlled by nucleotide-sensitive transcription start switching that produces transcripts with different potentials for translation. In Mycobacterium smegmatis and other bacteria, PyrR modulates translation of pyr genes by binding to their ribosome binding site. Evidence supporting these conclusions, generalizations for other bacteria, and prospects for future research are presented.

Regulation of pyrG expression in Bacillus subtilis: CTP-regulated antitermination and reiterative transcription with pyrG templates in vitro

The regulation of pyrG expression in a group of low GC Gram-positive bacteria was previously shown to be mediated by a novel form of transcription attenuation in which low levels of intracellular CTP induce reiterative addition of G residues at position +4 in the 5' end of the pyrG mRNA, which is encoded as pppGGGC. . . . The poly(G) sequences formed under these conditions act to prevent attenuation by base pairing with the C- and U-rich 5' strand of a downstream terminator stem-loop located in the pyrG leader. In this work we document the reconstitution of this regulatory system in vitro using only the native pyrG DNA template, RNA polymerase and appropriate concentrations of ribonucleotides. CTP-regulated reiterative transcription producing 5'-poly(G) tracts and regulation of transcription termination at the pyrG attenuator by CTP were demonstrated. Mutations in the native pyrG template that altered reiterative transcription and attenuation in vivo resulted in alternations in expression in the in vitro transcription system that were predicted by the mechanism described above. These findings provide strong experimental support for the proposed reiterative transcription/antitermination mechanism and confirm that no trans-acting regulatory protein is required for pyrG regulation.

The number of G residues in the Bacillus subtilis pyrG initially transcribed region governs reiterative transcription-mediated regulation

Derepression of pyrG expression in Bacillus subtilis involves CTP-sensitive reiterative transcription, which introduces up to 11 extra G residues at the 5' ends of pyrG transcripts. Insertion of three or more additional Gs at the 5' end of the pyrG initially transcribed region abolished reiterative transcription and caused constitutive expression.

Attenuation control of pyrG expression in Bacillus subtilis is mediated by CTP-sensitive reiterative transcription

In Bacillus subtilis and other Gram-positive bacteria, pyrimidine-mediated regulation of the pyrG gene, which encodes CTP synthetase, occurs through an attenuation mechanism involving an intrinsic transcription terminator in the pyrG leader region. Low intracellular levels of CTP prevent termination at the attenuator by a mechanism that requires the nontemplate strand sequence GGGC at the pyrG transcription initiation site (first G =+1) and the leader transcript sequence GCUCCC located at the 5' end of the terminator RNA hairpin. In this study, we demonstrate that reiterative transcription adds G residues (up to at least 10) to the 5' end of pyrG transcripts when B. subtilis cells are starved for pyrimidines but not when cells are grown with excess cytidine. Regulated repetitive addition of G residues, as well as pyrimidine-mediated pyrG regulation, requires the sequence GGGC or GGGT at the start of pyrG transcription. Mutational insertion of four extra G residues at the 5' end of the pyrG transcript (i.e., 5'-GGGGGGGC) results in constitutive pyrG expression. We propose that the incorporation of extra G residues by reiterative transcription at the wild-type promoter occurs when normal transcription elongation is stalled at position +4 by low levels of the incoming substrate, CTP, during pyrimidine limitation. The poly(G) extensions on the 5' ends of pyrG transcripts act to prevent transcription attenuation by base pairing with the sequence CUCCCUUUC located in the 5' strand of the terminator hairpin. This control mechanism is likely to operate in other Gram-positive bacteria containing similar pyrG leader sequences.

CTP limitation increases expression of CTP synthase in Lactococcus lactis

CTP synthase is encoded by the pyrG gene and catalyzes the conversion of UTP to CTP. A Lactococcus lactis pyrG mutant with a cytidine requirement was constructed, in which beta-galactosidase activity in a pyrG-lacLM transcriptional fusion was used to monitor gene expression of pyrG. A 10-fold decrease in the CTP pool induced by cytidine limitation was found to immediately increase expression of the L. lactis pyrG gene. The final level of expression of pyrG is 37-fold higher than the uninduced level. CTP limitation has pronounced effects on central cellular metabolism, and both RNA and protein syntheses are inhibited. Expression of pyrG responds only to the cellular level of CTP, since expression of pyrG has no correlation to alterations in UTP, GTP, and ATP pool sizes. In the untranslated pyrG leader sequence a potential terminator structure can be identified, and this structure is required for regulation of the pyrG gene. It is possible to fold the pyrG leader in an alternative structure that would prevent the formation of the terminator. We suggest a model for pyrG regulation in L. lactis, and probably in other gram-positive bacteria as well, in which pyrG expression is directly dependent on the CTP concentration through an attenuator mechanism. At normal CTP concentrations a terminator is preferentially formed in the pyrG leader, thereby reducing expression of CTP synthase. At low CTP concentrations the RNA polymerase pauses at a stretch of C residues in the pyrG leader, thereby allowing an antiterminator to form and transcription to proceed. This model therefore does not include any trans-acting protein for sensing the CTP concentration as previously proposed for Bacillus subtilis.