Expression of srnB gene of F plasmid by altered RNA polymerase in Escherichia coli

Localization of Escherichia coli rpoC mutations that affect RNA polymerase assembly and activity at high temperature

We localized five rpoC (beta') mutations affecting Escherichia coli RNA polymerase assembly. The Ts4, XH56, and R120 mutations changed beta' residues conserved throughout eubacteria; the JE10092 mutation occurred in the hypervariable region; rpoC1 (TsX) changed a universally conserved residue and corresponds to yeast rpb1-1. Thus, distinct, predominantly conserved beta' residues participate in interactions holding RNA polymerase together.

Topographic analysis of the toxic Gef protein from Escherichia coli

The chromosomal gef gene of Escherichia coli is a member of the gef gene family which encodes strongly toxic proteins of about 50 amino acids. We demonstrate here that the Gef protein is detectable by anti-peptide antibodies. Furthermore, we show that Gef is anchored in the cytoplasmic membrane by the N-terminal part of the protein, and that the C-terminal part is localized in the periplasm in a dimeric form with at least one disulphide bond. By mutagenesis of gef it is shown that the periplasmic portion of Gef encodes the toxic domain and that the dimerization of Gef is not essential for the toxic effect.

Mechanism of post-segregational killing by the hok/sok system of plasmid R1: sok antisense RNA regulates formation of a hok mRNA species correlated with killing of plasmid-free cells

The hok/sok system of plasmid R1, which mediates plasmid stabilization via killing of plasmid-free segregants, encodes two genes: hok and sok. The hok gene product is a potent cell-killing protein. The expression of hok is regulated post-transcriptionally by the sok gene-encoded repressor, an antisense RNA complementary to the hok mRNA leader region. We show here that the hok mRNA is very stable, while the sok RNA decays rapidly. We also observe a new hok mRNA species which is 70 nucleotides shorter in the 3'-end than the full-length hok transcript. The appearance of the truncated hok mRNA was found to be regulated by the sok antisense RNA. Furthermore, the presence of the truncated hok mRNA was found to be correlated with efficient expression of the Hok protein. On the basis of these findings, we propose an extended model in order to explain the killing of plasmid-free segregants by the hok/sok system.

Transcriptional regulation of F plasmid gene srnB: rifampicin-promoted in vitro readthrough of a terminator in the leader region

srnB is an F-plasmid encoded gene, otherwise silent, whose expression is induced by added rifampicin, leading to the release of cellular Mg2+ and degradation of stable RNA. In the absence of rifampicin, transcripts from the srnB gene were relatively short. S1 nuclease mapping revealed that the short mRNA species terminated within the leader, at the 3' end of a potential stem-and-loop structure. A deletion in the stem-loop resulted in constitutive synthesis of the mRNA that extended beyond the termination site into the structural gene. Even with the wild-type gene, transcription continued beyond the terminator sequence in the presence of added rifampicin. Most of the transcripts synthesized in the presence of rifampicin were long enough to encode the srnB protein. We hypothesize from these results that RNA polymerase associated with rifampicin can read through the terminator to induce srnB expression.

The pnd gene in E. coli plasmid R16: nucleotide sequence and gene expression leading to cell Mg2+ release and stable RNA degradation

The pnd gene promotes the degradation of stable RNA in the presence of rifampicin at 42 degrees C, but is repressed during normal growth (Ohnishi, Y. and Akimoto, S. (1980) J. Bacteriol. 144, 833-835). We have determined the sequence of a third srnB-pnd-type gene, and have analyzed the effects of its expression from an inducible promoter. The nucleotide sequence of the pnd gene of the R16 plasmid exhibits an open reading frame for a polypeptide with 50 amino-acid residues, with high sequence homology to the pnd gene of a plasmid (R483) of a different incompatibility group. A possible base-paired stem and loop structure, which may participate in the regulation of gene expression, was detected between the promoter and the initiation codon, analogous to that in two comparable genes, srnB in the F and pnd in the R483 plasmid. When bacterial cells containing a lac-pnd fusion plasmid were incubated with a lac inducer at 30 degrees C, magnesium was released from the cells in bulk, and spheroplasts of the cells lysed even in hypertonic solution. Furthermore, when Mg2+ efflux was inhibited in the medium containing 5 mM Mg2+ or in Tris-HCl buffer, the degradation of stable RNA at 42 degrees C was inhibited. These results suggest that expression of the pnd gene effects a release of cellular magnesium by a membrane alterations, resulting in the stable RNA degradation at a higher temperature.

Nucleotide sequence of the pnd gene in plasmid R483 and role of the pnd gene product in plasmolysis

The pnd gene of R plasmid R483, like the srnB gene of the F plasmid, increases the degradation of stable RNA in Escherichia coli. The nucleotide sequence of the pnd locus was determined and compared with that of the srnB locus. The genes have open reading frames that are 54% homologous, and both have an upstream inverted repeat sequence. The pnd gene expression seems to decrease the osmotic barrier of the cytoplasmic membrane, since no plasmolytic vacuoles were formed in the cells carrying the gene when the cells were exposed to hypertonic sucrose solution. This result suggests that RNase I in the periplasm passes through the altered membrane to degrade stable RNA in the cytoplasm.

Plasmid genes increase membrane permeability in Escherichia coli

The membrane permeability to o-nitrophenyl beta-D-galactoside is increased in the presence of rifampicin in Escherichia coli cells carrying srnB+ or pnd+ plasmids, but not in the cells carrying srnB- or pnd- mutant plasmids. The same permeability alteration was also observed at 42 degrees C when a rpoC4- mutant strain was used as a host strain in the absence of rifampicin. These results and the blockage of the effects by action of chloramphenicol suggest that the increase of permeability to o-nitrophenyl galactoside was caused by the expression of srnB+ or pnd+ gene, respectively. srnB+ gene expression leads to massive RNA degradation, probably through the activation of the rna+ gene product. In an rna- strain carrying the srnB+ plasmid, the extent of RNA degradation was reduced, whereas the permeability to o-nitrophenyl galactoside was increased to the same level as in the rna+ strain. Also, the increase in permeability to o-nitrophenyl galactoside was observed at 30 degrees C, although high-temperature incubation (42 degrees C) was necessary for the induction of RNA degradation. These results suggest that the alteration in permeability is a more direct effect of the expression of srnB+ or pnd+ gene and that the RNA degradation is a secondary phenomenon caused by the alteration in the membrane.

Cloning and expression of the pnd gene of R16: determination of transcriptional direction and evolutionary analysis

The gene promoting nucleic-acid degradation (pnd) of IncB plasmid R16 was cloned into the vector plasmid pACYC177. The pnd gene was found to be located on a 0.55-kilobase (kb) AluI-PstI fragment by constructing subclones carrying various portions of the initially cloned fragment. The direction of transcription of the pnd gene was determined by inserting the gene in both orientations into the lacZ' gene of the plasmid pUR222. In the recombinant plasmid pCM2, transcription of the pnd gene was controlled by the lac promoter region. Addition of cAMP at 42 C resulted in rapid degradation of stable RNA in cells harboring pCM2. In contrast, no RNA degradation was observed in cells harboring pCM14, which has the same insert as pCM2 but in the opposite orientation. The equivalent gene, pnd of IncI alpha plasmid R483, has previously been cloned, and a detailed restriction map of the region has been constructed (Akimoto, S., and Ohnishi, Y. 1982. Microbiol. Immunol. 26: 779-793). We constructed a detailed restriction map of the pnd region of R16 and compared it with that of R483. Restriction analyses revealed a similar structure in these two pnd regions. The results suggest that the pnd genes of R16 and R483 have a common evolutional origin.