Initial trp operon sequence in Escherichia coli is transcribed without coupling to translation

lac Transcription in Escherichia coli cells treated with chloramphenicol

When protein synthesis was blocked by chloramphenicol in vivo, transcription initiation of lac mRNA was severely inhibited. In a promoter mutant (L8-UV5) or in wild-type cells supplemented with adenosine 3',5'-phosphate (greater than or equal to 5 mM), nearly normal initiation could be achieved, and when the mRNA chains formed were extracted, they coded for the 5'-terminal alpha-peptide of the lacZ gene in vitro. However, even under such conditions, only a fraction of RNA polymerases proceeded to the end of the Z gene in the presence of chloramphenicol; as a consequence, a wide range of sizes of mRNA was produced, and very few transcripts were formed all the way to the natural termination site of the operon. In other words, premature transcription termination occurred in chloramphenicol-treated cells, as current models predict, but terminations occurred to variable extents at several intragenic sites and apparently at least one intergenic site. Termination at intragenic sites occurred far less in cells bearing a mutation in the transcription termination factor rho.

Involvement of the nusB gene products in transcription of Escherichia coli tryptophan operon in vitro

Synthesis of trp mRNA in vitro directed by plasmid DNA carrying the entire trp operon was studied using crude protein extracts (S-100) from Escherichia coli strains carrying the nusA or nusB mutation or both. It was found that the levels of trp mRNA transcribed from the promoter-distal genes (trpCBA) relative to that from the promoter-proximal genes (trpED) was markedly lower with extracts from the nus- mutants than that from the nus+ strain. Kinetic experiments suggest that termination of RNA transcripts at intragenic transcriptional barriers is prevented by the nus gene products from allowing efficient expression of the operon.

Comparative studies of the effect of DNA superhelicity on in vitro transcription catalyzed by Escherichia coli S100 proteins and purified RNA polymerase

The effect of DNA superhelicity on in vitro transcription catalyzed by purified Escherichia coli RNA polymerase or S100 crude extract proteins was examined at various KCl concentrations. DNA from a recombinant plasmid pMT48 harboring the pL promotor-controlled fused N-trp genes and the pR promotor-controlled tof (cro) gene was employed as a template. Stimulation of transcription by superhelicity is generally more pronounced with the S100 crude extract proteins than with pure RNA polymerase. At KCl concentrations lower than 100 mM with pure RNA polymerase, there is no significant difference in the template activity between the supercoiled and relaxed forms of pMT48 DNA. In contrast, the dependence of efficient template activity on superhelicity is great over a whole range of KCl concentrations from 1.7 to 400 mM in the system using the S100 crude extract. The relative insensitivity of the pR promotor to superhelicity can be observed in either transcription assay system. Analysis of the kinetics of pL-promoted synthesis of trp mRNA indicates that diminished transcription in vitro on a relaxed template results mainly from less frequent RNA chain initiations, but at least in part from premature arrest of the chain elongation.

In vivo correlation between DNA supercoiling and transcription

The superhelical density of pMT plasmid DNA in Escherichia coli cells was measured as a function of the transcriptional activity, which was reduced by treatment with coumermycin or oxolinic acid. Superhelicity was assayed by agarose gel electrophoresis of DNA extracted from cells gently lysed by sarkosyl. Coumermycin treatment reduced the proportion of supercoiled plasmid DNA in concert with a decrease in the rate of plasmid-coded synthesis of trp mRNA, implying a correlation between supercoiling of DNA and its suitability for transcription. On the other hand, the oxolinic acid-induced loss of supercoiled plasmid DNA was relatively small, while concomitant inhibition of trp mRNA synthesis was very severe. Treatment of the cells with these two drugs never removed all of supertwists from the pMT plasmids carried.

Evidence for endonucleolytic cleavage at the 5'-proximal segment of the trp messenger RNA in Escherichia coli

The 5'-proximal trp leader RNA segment (about 5S) decays at 2 to 3 times slower rates than the distal trp mRNA sequence. This has been demonstrated by employing the deletion mutants which lack a large portion of the structural genes but retain the promoter-proximal region of the trp operon. Relative stability of the leader RNA is not merely due to the presence of an untranslatable region in the segment; the internal untranslatable segment of trp mRNA downstream from the nonsense alteration site of a double mutant trpAD28.trpE9758 decays as fast as the normal trp mRNA sequence. These results suggest that the trp mRNA is endonucleolytically cleaved to yield the small 5'-proximal leader RNA segment before the distal mRNA decays and that the leader RNA sequence is not subject to usual mode of mRNA decay in the 5' to 3' direction.

PL-promoted transcription of the promoter-proximal N-trp region is insensitive to chloramphenicol in the absence of N function

When the trp operon is translocated into the early region of lambda phage, transcription originated at the PL promotor is known to be modified by function of the N gene product so that transcription of the operon continues when translation is blocked by nonsense mutations or by ribosomal antibiotics. When N function is deficient in a phage that joins the trp operon to a point distal to the N gene, deleting the tL site, nonsense mutations (Franklin, 1974) or chloramphenicol (Nakamura et al., accompanying paper) again block transcription of the bacterial operon. However, here we report that transcription over about the first 800 nucleotide pairs starting from the PL promotor of the N-trp operon is still insensitive to chloramphenicol even in the absence of N function. The region covers the full N gene and the initial bit of the trp operon.

Gene affecting longevity of messenger RNA: a mutant of Escherichia coli with altered mRNA stability

We have screened 897 temperature sensitive growth mutants of E. coli for mutant strains showing longer mRNA half-life. The fate of pulse-labelled RNA was examined at 42 degrees C after cessation of RNA synthesis and with prior exposure to nonpermissive temperature (42 degrees C). Eight stains showed altered turnover of RNA (presumably mRNA), and further analysis on mutant strain JE15144 indicated that the stability of pulse-labeled RNA as well as of tryptophan (trp) mRNA increased four to seven fold over its parental strain at 42 degrees C. At 4 min or 10 min after addition of rifampicin, some 70 to 80% of polyribosome in the growing cells could still be conserved in JE15144 cultured at the nonpermissive temperature while little, if any, polyribosomes remained in its parental strain (PA3092) under the same condition. Two generation times were required for complete stoppage of growth of this mutant strain after shifting to 42 degrees C, and protein synthesis continued at a significant, but slightly reduced, rate at 42 degrees C. However, functional decay of mRNA in the mutant strain, with respect to the capacity for producing peptides, appeared to be similar to the parent strain, with half-lives of 3.5 min in PA3092 and 4.7 min in JE15144.

In vitro transcription of the tryptophan operon in isolated bacterial nucleoids

In vitro transcription of the trp operon in isolated nucleoids from Escherichia coli was studied. RNA synthesis in this system occurred primarily as a continuation of transcription which had been initiated in vivo; little or no initiation of new RNA chains was observed. Transcription of the trp operon in nucleoids by endogenous RNA polymerase procedded efficiently and ceases sequentially in the order of the gene sequence within the operon. Under these conditions, no appreciable exonuccleolytic digestion of nascent 3H-RNA was found, though some endonucleolytic cleavage was generally seen. Little or no incorporation of 14C-leucine into polypeptides was observed, inspite of tha fact that considerable number of ribosomes and nascent RNA chains were found attached to the isolated nucleoids. The synthesis of trp mRNA continued in the presence of chloramphenicol or fusidic acid, or under conditions where the rebosomal translocation factor G was inactivated. From these and other kinetic studies of trp mRNA synthesis in nucleoids obtained from nonsense strong polar mutants of the trp operon, it was shown that transcription in nucleoids was not connected functionally with transloational processes and thus unable to exhibit polarity effected by a nonsense mutation or by general translational blockage. In studies employing nucleoids from nonsense strong polar mutants of the trp operon, it was demonstrated that RNA polymerase are scantily distributed over the region downstream from the nonsense mutation site of the operon, thereby supporting a notion that in vivo transcription is eventually terminated near the nonsense mutation.