Inability to detect RNase V in Escherichia coli and comparison of other ribonucleases before and after infection with coliphage T7

Killer and protective ribosomes

In prokaryotes, translation influences mRNA decay. The breakdown of most Escherichia coli mRNAs is initiated by RNase E, a 5'-dependent endonuclease. Some mRNAs are protected by ribosomes even if these are located far upstream of cleavage sites ("protection at a distance"), whereas others require direct shielding of these sites. I argue that these situations reflect different modes of interaction of RNase E with mRNAs. Protection at a distance is most impressive in Bacilli, where ribosomes can protect kilobases of unstable downstream sequences. I propose that this protection reflects the role in mRNA decay of RNase J1, a 5'-->3' exonuclease with no E. coli equivalent. Finally, recent years have shown that besides their protective role, ribosomes can also cleave their mRNA under circumstances that cause ribosome stalling. The endonuclease associated with this "killing" activity, which has a eukaryotic counterpart ("no-go decay"), is not characterized; it may be borne by the distressed ribosome itself.

Mechanism of erythromycin-induced ermC mRNA stability in Bacillus subtilis

In Bacillus subtilis, the ermC gene encodes an mRNA that is unusually stable (40-min half-life) in the presence of erythromycin, an inducer of ermC gene expression. A requirement for this induced mRNA stability is a ribosome stalled in the ermC leader region. This property of ermC mRNA was used to study the decay of mRNA in B. subtilis. Using constructs in which the ribosome stall site was internal rather than at the 5' end of the message, we show that ribosome stalling provides stability to sequences downstream but not upstream of the ribosome stall site. Our results indicate that ermC mRNA is degraded by a ribonucleolytic activity that begins at the 5' end and degrades the message in a 5'-to-3' direction.

A microsomal exoribonuclease from rat liver

A exoribonuclease has been purified from the microsomes of rat liver. The enzyme had an apparent molecular weight of 80 000-83 000 and produced, via a processive mechanism, 5'-AMP as the only product from poly(A). The degradation was found to proceed in the 3' to 5' direction. The relative rates of breakdown of synthetic polynucleotides by the enzyme under standard conditions were in the order poly(A) equal to poly(U) greater than poly(C). In addition to Mg2+, K+ was required for maximum activity. The enzymic activity was inhibited by p-chloromercuribenzoate and poly(G), but not by a rate liver RNAase inhibitor. The effect of spermine on the breakdown of synthetic polynucleotides by the enzyme has been studied. In the absence of K+, the breakdown of poly(C) was stimulated and that of poly(A) was stimulated slightly. However, the breakdown of poly(U) was inhibited slightly by spermine.

Translation and mRNA decay

Degradation of messenger RNA from the lactose operon (lac mRNA) was measured during the inhibition of protein synthesis by chloramphenicol (CM) or of translation-initiation by kasugamycin (KAS). With increasing CM concentration mRNA decay becomes slower, but there is no direct proportionality between rates of chemical decay and polypeptide synthesis. During exponential growth lac mRNA is cleaved endonucleolytically (Blundell and Kennell, 1974). At a CM concentration which completely inhibits all polypeptide synthesis this cleavage is blocked. In contrast, if only the initiation of translation is blocked by addition of KAS, the cleavage rate as well as the rate of chemical decay are increased significantly without delay. These faster rates do not result from immediate degradation of the lengthening stretch of ribosome-free proximal message, since the full-length size is present and the same discrete message sizes are generated during inhibition. These results suggest that neither ribosomes nor translation play an active role in the degradative process. Rather, targets can be protected by the proximity of a ribosome, and without nearly ribosomes the probability of cleavage becomes very high. During normal growth there is a certain probability that any message is in such a vulnerable state, and the fraction of vulnerable molecules determines the inactivation rate of that species.

Possible mechanism of E. coli messenger RNA degration: non-enzymatic degradation

The present paper points out lack of evidence to support the presently prevailing concept that E. coli mRNA turnover, in the gene expression process, cannot take place without mRNase(s). The present paper draws attention to possible physicochemical factors involved in the degradation, and advances a notion of non-enzymatic spontaneous degradation of E. coli mRNA in its expression process. This suggested hypothesis helps to explain hitherto reported findings on the mode of E. coli mRNA degradation.

The role of ribonuclease II in the maturation of precursor 16S ribosomal ribonucleic acid in Escherichia coli

The suggested involvement of ribonuclease II in the maturation of rRNA has been examined directly by determining the activity of the enzyme and the amount of p16S rRNA in cell-free extracts from Escherichia coli A19 and its temperature-sensitive derivative N464 grown under experimental conditions designed to vary the amounts of enzyme and precursor independently. In strain A19 the enzyme showed maximum activity in circumstances where the amount of p16S rRNA was normal (e.g. exponential-phase cells) or raised eight times (e.g. during inhibition of growth by methionine starvation of the relaxed auxotroph or by chloramphenicol or puromycin treatment). In strain N464 at the non-permissive temperature the ribonuclease II activity may be decreased by 50% without effect upon the amount of p16S rRNA, whereas in methionine starvation of this strain the enzyme activity is at a maximum and the p16S rRNA is eight times that in exponential-phase cells. These observations are discussed in relation to the previously implied role of ribonuclease II in the maturation of rRNA within ribosome precursors.

The control of ribonucleic acid synthesis in bacteria. Fluctuations in messenger ribonucleic acid synthesis in cultures recovering from amino acid starvation

Changes in the cell content and rate of synthesis of mRNA were studied in auxotrophs of Escherichia coli recovering from a period of amino acid deprivation. Parallel studies were carried out on bacterial strains inhibited with trimethoprim, when glycine and methionine were added to relieve an amino acid deficiency. In the latter case, protein synthesis was still severely inhibited through a lack of N-formylmethionyl-tRNA(fMet) for chain initiation, so that fewer ribosomes were attached to mRNA chains. (1) In RC(str) strains recovering from amino acid starvation, there was a transient oversynthesis of mRNA, but the amounts returned to normal after about a 15-min period of recovery. RC(rel) strains did not show this effect; any extra mRNA accumulated during the previous starvation period was rapidly lost, but no oversynthesis occurred during the resumption of growth. (2) In trimethoprim-inhibited cultures supplemented with glycine and methionine, mRNA was produced at the same rate, relative to stable RNA species, as during normal growth. The evidence implied that decreased rates of ribosome attachment had no effect on the functional or chemical lifetime of the mRNA fraction. This suggests that mRNA stability does not depend on the frequency of translation by ribosomes. (3) Changes in the mRNA contents of trimethoprim-inhibited RC(str) and RC(rel) cultures were noted soon after supplementation with glycine and methionine. These closely followed those observed in cultures recovering from simple amino acid withdrawal.

Messenger ribonucleic acid synthesis and degradation in Escherichia coli during inhibition of translation

Various aspects of the coupling between the movement of ribosomes along messenger ribonucleic acids (mRNA) and the synthesis and degradation of mRNA have been investigated. Decreasing the rate of movement of ribosomes along an mRNA does not affect the rate of movement of some, and possibly most, of the RNA polymerases transcribing the gene coding for that mRNA. Inhibiting translation with antibiotics such as chloramphenicol, tetracycline, or fusidic acid protects extant mRNA from degradation, presumably by immobilizing ribosomes, whereas puromycin exposes mRNA to more rapid degradation than normal. The promoter distal (3') portion of mRNA, synthesized after ribosomes have been immobilized by chloramphenicol on the promoter proximal (5') portion of the mRNA, is subsequently degraded.

Salmonella typhimurium mutants lacking ribonuclease I: effect on the polarity of histidine mutants

Mutants of Salmonella typhimurium containing 1 to 2% of wild-type ribonuclease I activity were isolated. The rns mutation had no effect on the polarity of mutations in the S. typhimurium histidine operon. Even in the presence of an rns mutation, it was not possible to obtain strong suppressors of the polarity of two polar mutations in the his operon.

Ribonuclease II activity in a "presumptive" ribonuclease II-deficient mutant

A mutant strain of Escherichia coli previously thought to possess low levels of ribonuclease II activity has normal levels of ribonuclease II after partial purification of this enzyme from crude extracts.