Escherichia coli endoribonucleases involved in cleavage of bacteriophage T4 mRNAs

The dmd mutant of bacteriophage T4 has a defect in growth because of rapid degradation of late-gene mRNAs, presumably caused by mutant-specific cleavages of RNA. Some such cleavages can occur in an allele-specific manner, depending on the translatability of RNA or the presence of a termination codon. Other cleavages are independent of translation. In the present study, by introducing plasmids carrying various soc alleles, we could detect cleavages of soc RNA in uninfected cells identical to those found in dmd mutant-infected cells. We isolated five Escherichia coli mutant strains in which the dmd mutant was able to grow. One of these strains completely suppressed the dmd mutant-specific cleavages of soc RNA. The loci of the E. coli mutations and the effects of mutations in known RNase-encoding genes suggested that an RNA cleavage activity causing the dmd mutant-specific mRNA degradation is attributable to a novel RNase. In addition, we present evidence that 5'-truncated soc RNA, a stable form in T4-infected cells regardless of the presence of a dmd mutation, is generated by RNase E.     

A novel endoribonuclease, RNase LS, in Escherichia coli

The dmd gene of bacteriophage T4 is required for the stability of late-gene mRNAs. When this gene is mutated, late genes are globally silenced because of rapid degradation of their mRNAs. Our previous work suggested that a novel Escherichia coli endonuclease, RNase LS, is responsible for the rapid degradation of mRNAs. In this study, we demonstrated that rnlA (formerly yfjN) is essential for RNase LS activity both in vivo and in vitro. In addition, we investigated a role of RNase LS in the RNA metabolism of E. coli cells under vegetative growth conditions. A mutation in rnlA reduced the decay rate of many E. coli mRNAs, although there are differences in the mutational effects on the stabilization of different mRNAs. In addition, we found that a 307-nucleotide fragment with an internal sequence of 23S rRNA accumulated to a high level in rnlA mutant cells. These results strongly suggest that RNase LS plays a role in the RNA metabolism of E. coli as well as phage T4.     

Opposite roles of the dmd gene in the control of RNase E and RNase LS activities

When the dmd gene of bacteriophage T4 is defective, expression of middle genes starts normally but drops abruptly. However, the residual expression of middle genes at late stages continues at a higher rate in cells infected with a dmd mutant than with the wild type. In order to understand the complex effects of the dmd gene, we followed changes in the quantity of mRNA from a middle gene, uvsY. The uvsY mRNA was degraded rapidly by RNase LS at middle stages but stabilized at late stages, suggesting that RNase LS targets middle-gene mRNAs only at middle stages. Furthermore, another RNase targeting middle mRNAs at late stages is also suggested to be inactivated when dmd is mutated. We found that RNase E was involved in the degradation of uvsY mRNA. Judging from the processing of gene-32 mRNA, RNase E activity declines after the beginning of the middle stage when dmd is defective.     

RNA cleavage linked with ribosomal action

Ribonuclease LS in Escherichia coli is a potential antagonist of bacteriophage T4. When T4 dmd is mutated, this RNase efficiently cleaves T4 mRNAs and leads to the silencing of late genes, thus blocking T4 growth. We previously found that, when two consecutive ochre codons were placed in the open reading frame of T4 soc, RNase LS cleaved soc mRNA at a specific site downstream of the ochre codons. Here, we demonstrate that RNase LS cleaves soc RNA at the same site even when only a single ochre codon is present or is replaced with either an amber or an opal codon. On the other hand, disruption of the Shine-Dalgarno sequence, a ribosome-binding site required for the initiation of translation, eliminates the cleavage. These results strongly suggest that RNase LS cleaves in a manner dependent on translation termination. Consistent with this suggestion, the cleavage dependency on an amber codon was considerably reduced in the presence of amber-codon-suppressing tRNA. Instead, two other cleavages that depend on translation of the region containing the target sites occurred farther downstream. Additional analysis suggests that an interaction of the ribosome with a stop codon might affect the site of cleavage by RNase LS in an mRNA molecule. This effect of the ribosome could reflect remodeling of the high-order structure of the mRNA molecule.     

Escherichia coli rnlA and rnlB compose a novel toxin-antitoxin system

RNase LS was originally identified as a potential antagonist of bacteriophage T4 infection. When T4 dmd is defective, RNase LS activity rapidly increases after T4 infection and cleaves T4 mRNAs to antagonize T4 reproduction. Here we show that rnlA, a structural gene of RNase LS, encodes a novel toxin, and that rnlB (formally yfjO), located immediately downstream of rnlA, encodes an antitoxin against RnlA. Ectopic expression of RnlA caused inhibition of cell growth and rapid degradation of mRNAs in ΔrnlAB cells. On the other hand, RnlB neutralized these RnlA effects. Furthermore, overexpression of RnlB in wild-type cells could completely suppress the growth defect of a T4 dmd mutant, that is, excess RnlB inhibited RNase LS activity. Pull-down analysis showed a specific interaction between RnlA and RnlB. Compared to RnlA, RnlB was extremely unstable, being degraded by ClpXP and Lon proteases, and this instability may increase RNase LS activity after T4 infection. All of these results suggested that rnlA-rnlB define a new toxin-antitoxin (TA) system.     

Scarce adenylation in bacteriophage T4 mRNAs

The degradation of mRNA is crucial for the rapid shift of bacteriophage T4 gene expression from early to late. The present study was conducted to investigate whether T4 mRNA is polyadenylated or not, because polyadenylation is known to facilitate the degradation of mRNA in Escherichia coli cells. Total RNA extracted from T4-infected cells was subjected to self-circularization or intermolecular ligation by T4 RNA ligase, and a region containing the 3'-5' junction was amplified by RT-PCR. Cloning and sequencing as well as the length distribution of amplified DNA fragments revealed no adenines at the 3'-ends of uvsY and soc RNAs. The present result suggests that T4 mRNA is not significantly adenylated.     

Negative effect of glucose on ompA mRNA stability: a potential role of cyclic AMP in the repression of hfq in Escherichia coli

Glucose is a carbon source that is capable of modulating the level of cyclic AMP (cAMP)-regulated genes. In the present study, we found that the stability of ompA mRNA was reduced in Escherichia coli when glucose (40 mM) was present in Luria-Bertani (LB) medium. This effect was associated with a low level of cAMP induced by the glucose. The results were confirmed with an adenylyl cyclase mutant with low levels of cAMP that are not modulated by glucose. Northern blot and Western blot analyses revealed that the host factor I (Hfq) (both mRNA and protein) levels were downregulated in the presence of cAMP. Furthermore, we showed that a complex of cAMP receptor protein (CRP) and cAMP binds to a specific P3(hfq) promoter region of hfq and regulates hfq expression. The regulation of the hfq gene was confirmed in vivo using an hfq-deficient mutant transformed with an exogenous hfq gene containing the promoter. These results demonstrated that expression of hfq was repressed by the CRP-cAMP complex. The presence of glucose resulted in increased Hfq protein levels, which decreased ompA mRNA stability. An additional experiment showed that cAMP also increased the stability of fur mRNA. Taken together, these results suggested that the repression of Hfq by cAMP may contribute to the stability of other mRNA in E. coli.     

A role of RnlA in the RNase LS activity from Escherichia coli

Escherichia coli ribonuclease LS is a potential antagonist of bacteriophage T4. When the T4 dmd gene is defective, RNase LS cleaves T4 mRNAs and antagonizes T4 reproduction. Our previous work demonstrated that E. coli rnlA is essential for RNase LS activity. Here we show that His-tagged RnlA cleaves T4 soc RNA at one of the sites also cleaved by RNase LS in a cell extract. The cleavage activities of His-tagged RnlA and the RNase LS activity in a cell extract were inhibited by Dmd encoded by T4 phage. Fractionation of the RNase LS activity in a cell extract showed that it sedimented through a sucrose density gradient as a 1000-kDa complex that included RnlA. Pull-down experiments revealed more than 10 proteins associated with His-tagged RnlA. Among these, triose phosphate isomerase exhibited a remarkable affinity to RnlA. These results suggest that RnlA plays a central role in RNase LS activity and that its activity is regulated by multiple components.     

Hfq Influences Multiple Transport Systems and Virulence in the Plant Pathogen Agrobacterium tumefaciens

The Hfq protein mediates gene regulation by small RNAs (sRNAs) in about 50% of all bacteria. Depending on the species, phenotypic defects of an hfq mutant range from mild to severe. Here, we document that the purified Hfq protein of the plant pathogen and natural genetic engineer Agrobacterium tumefaciens binds to the previously described sRNA AbcR1 and its target mRNA atu2422, which codes for the substrate binding protein of an ABC transporter taking up proline and γ-aminobutyric acid (GABA). Several other ABC transporter components were overproduced in an hfq mutant compared to their levels in the parental strain, suggesting that Hfq plays a major role in controlling the uptake systems and metabolic versatility of A. tumefaciens. The hfq mutant showed delayed growth, altered cell morphology, and reduced motility. Although the DNA-transferring type IV secretion system was produced, tumor formation by the mutant strain was attenuated, demonstrating an important contribution of Hfq to plant transformation by A. tumefaciens.     

Poliovirus-mediated disruption of cytoplasmic processing bodies

Metazoan cells form cytoplasmic mRNA granules such as stress granules (SG) and processing bodies (P bodies) that are proposed to be sites of aggregated, translationally silenced mRNAs and mRNA degradation. Poliovirus (PV) is a plus-strand RNA virus containing a genome that is a functional mRNA; thus, we investigated if PV antagonizes the processes that lead to formation of these structures. We have previously shown that PV infection inhibits the ability of cells to form stress granules by cleaving RasGAP-SH3-binding protein (G3BP). Here, we show that P bodies are also disrupted during PV infection in cells by 4 h postinfection. The disruption of P bodies is more rapid and more complete than disruption of stress granules. The kinetics of P body disruption correlated with production of viral proteinases and required substantial viral gene product expression. The organizing mechanism that forms P body foci in cells is unknown; however, potential scaffolding, aggregating, or other regulatory proteins found in P bodies were investigated for degradation. Two factors involved in 5'-end mRNA decapping and degradation, Xrn1 and Dcp1a, and the 3' deadenylase complex component Pan3 underwent accelerated degradation during infection, and Dcp1a may be a direct substrate of PV 3C proteinase. Several other key factors proposed to be essential for P body formation, GW182, Edc3, and Edc4, were unaffected by poliovirus infection. Since deadenylation has been reported to be required for P body formation, viral inhibition of deadenylation, through Pan3 degradation, is a potential mechanism of P body disruption.     

NAD(+) consumption in carcinogen-treated hamster cells overexpressing a dominant negative mutant of poly(ADP-ribose) polymerase

We found that 82- and 76-kDa proteins in the outer-membrane fractions were overproduced in the hfq::cat mutant cells when grown in synthetic media. Expression of these proteins was repressed by addition of FeCl(3) in the mutant as well as in the wild type. It was revealed that these are FepA and FhuE proteins involved in iron transport. The hfq::cat mutant was more susceptible to killing by hydrogen peroxide, probably due to the excess incorporation of iron, which potentially generates hydroxyl radicals. Increased incorporation of iron in the hfq::cat mutant was also confirmed by the suppressive effect on the ftsH1 mutation. These results suggest that the hfq gene product is involved in the defense mechanism against oxidative stress.