Regulation of RNA polymerase synthesis in Escherichia coli: a mutant unable to synthesize the enzyme at 43 degrees

Chemistry and biology of E. coli ribosomal protein L12

E. coli ribosomal protein L12, because of its unique features, has been studied in more detail than perhaps any of the other ribosomal proteins. Unlike the other ribosomal proteins that are generally present in stoichiometric amounts, there are four copies of L12 per ribosome, some of which are acetylated on the N-terminal serine. The acetylated species, referred to as L7, has not been shown, as yet, to possess any different biological activity than L12. A specific enzyme that acetylates L12 to form L7, using acetyl-CoA as the acetyl donor, has been purified from E. coli extracts. L12 is also unique in that it does not contain cysteine, tryptophan, histidine, or tyrosine, is very acidic (pI: 4.85) and has a high content of ordered secondary structure (approximately 50%). The protein is normally found in solution as a dimer and also forms a tight complex with ribosomal protein L10. There are three methionine residues in L12, located in the N-terminal region of the protein, one or more of which are essential for biological activity. Oxidation of the methionines to methionine sulfoxide prevents dimer formation and inactivates the protein. The four copies of L12 are located in the crest region(s) of the 50S ribosomal subunit. There is good evidence that the soluble factors, such as IF-2, EF-Tu, EF-G and RF, interact with L12 on the ribosome during the process of protein synthesis. This interaction is essential for the proper functioning of each of the factors and for GTP hydrolysis associated with the individual partial reactions of protein synthesis. The L12 gene is located on an operon that contains the genes for L10 and beta beta' subunits of RNA polymerase at about 88 min on the bacterial chromosome. DNA-directed in vitro systems have been used to study the unique regulation of the expression of these genes. Autogenous regulation, translational control, and transcription attenuation are regulatory mechanisms that function to control the synthesis of these proteins.

Autogenous and post-transcriptional regulation of RNA polymerase synthesis

The regulation of gene expression was studied, for the Escherichia coli rpoBC operon, which includes the genes, rpoB and rpoC, for the beta and beta subunits of RNA polymerase, and rplJ and rplL, for the two proteins, L10 and L7/12, of the 50S ribosome. The gene organization agrees well with the accumulated observations indicating the coordinate synthesis of RNA polymerase and ribosomes under various growth conditions for wild-type E. coli cells. On the other hand, the differential regulation of the two essential components observed under restrictive growth conditions, after addition of various drugs or with certain mutants, in particular those carrying mutations in the RNA polymerase genes, was found to take place through two novel regulation systems: The transcriptional termination at an internal attenuation site and the two autogenous and posttranscriptional controls, being specific for the two ribosomal protein genes and the two RNA polymerase subunit genes, respectively. The majority of the transcription initiated from the promoter rpoP beta terminates at an attenuator site between the promoter-proximal rplJL and the promoter-distal rpoBC genes. The frequency of the attenuation seems to control the relative level of RNA polymerase synthesis to that of ribosomes. The expression of rpoBC genes is subject to an autogenous regulation, in which both RNA polymerase holoenzyme and alpha 2 beta complex function as regulatory molecules with repressor activity. The autogenous regulation was found to operate at post-transcriptional step(s), probably at the level of translation. During the study on the regulation of RNA polymerase synthesis, we noticed that the rpoBC operon contained another autogenous regulation circuit, in which the synthesis of L10 and L7/12 was specifically repressed by the L10-L7/12 complex. Molecular mechanisms and physiological meanings of the novel regulations are discussed.

High efficiency temperature-sensitive amber suppressor strains of Escherichia coli K12: construction and characterization of recombinant strains with suppressor-enhancing mutations

A set of eight strains combining the supD43,74 ts1 suppressor gene with alleles of three suppressor-enhancing (sue) genes have been constructed and characterized. The sue mutations work cooperatively to raise suppressor activity and together raise the activity of the supD43,74-encoded suppressor 40-fold. These strains further expand the utility of the ts suppressor system by providing as much as 100% suppressor activity at temperatures at or below 20 degrees C to as little as 0.015% suppressor activity at 43 degrees C.

High efficiency temperature-sensitive amber suppressor strains of Escherichia coli K12: isolation of strains with suppressor-enhancing mutations

Two independent high efficiency ts amber suppressor strains have been isolated as derivatives of a well-characterized supD ts suppressor strain (Oeschger and Woods, 1976). The mutations which raise suppressor activity have been shown by Hfr mapping to be distinct from each other and the supD locus. One of the isolates provides up to 100% efficient suppressor activity at low temperatures.

Gene expression in Escherichia coli B/r during partial rifampicin-mediated restrictions of transcription initiation

The antibiotic rifampicin inhibits transcription initiation, but not the elongation and completion of nascent RNA transcripts. Addition of low concentrations of rifampicin only partially blocks initiation but at the same time specifically alters the general pattern of transcription in the culture. The transcription of genes specifying the beta and beta' subunits of RNA polymerase, and to a lesser extent of the genes specifying the RNA and protein components of the ribosome, was specifically stimulated relative to total transcription. In contrast, the transcription of the lactose operon was selectively reduced. These results are consistent with the ideas that the level of expression of the genes specifying the beta and beta' subunits is sensitive to the general rate of RNA synthesis in the culture, and that the expression of the beta and beta' RNA polymerase genes is related to the expression of ribosome component genes.

Rich culture medium for the radiochemical labeling of proteins and nucleic acids

Yeast extract was treated with tyrosine decarboxylase and used to prepare a rich, complex medium virtually free of tyrosine. The medium supported maximal growth rates for Escherichia coli prototrophs, as well as for defined and undefined auxotrophs. It has made possible the efficient radiochemical labeling of cells growing optimally in complex medium and the characterization of mutants with undefined requirements. Similarly prepared media may be useful for the study of fastidious organisms and organisms for which no defined medium has been described.

Temperature-sensitive RNA polymerase II mutations in Chinese hamster ovary cells

Mutant Chinese hamster ovary cell lines temperature-sensitive (TS) for growth and containing TS mutations in RNA polymerase II (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6) have been isolated. Wild-type cells were treated with the mutagen N-methyl-N'-nitro-N-nitrosoguanidine and a population of cells possessing mutations in RNA polymerase II was initially selected by isolating alpha-amanitin-resistant clones at 34 degrees . Of 168 such alpha-amanitin-resistant isolates screened for temperature sensitivity, nine were TS for growth at 39.5 degrees . By examining the behavior of the alpha-amanitin resistance of these TS cell lines in somatic cell hybrids, the TS mutation in a number of them was shown to be in RNA polymerase II. Hybrid cells obtained by the fusion of the TS and alpha-amanitin-resistant cells with cells possessing alpha-amanitin-sensitive polymerase II grew at both 34 degrees and 39.5 degrees ; the TS mutations were recessive. At 34 degrees all the hybrids were alpha-amanitin-resistant and possessed a mixture of alpha-amanitin-resistant and sensitive polymerase II. At 39.5 degrees the alpha-amanitin-resistant polymerase II activities in hybrids of four of the TS cell lines were lost; these four lines were alpha-amanitin-sensitive and possessed only alpha-amanitin-sensitive polymerase II. Temperature-insensitive revertants of two of these mutants were isolated. Reversion of the TS phenotype for mutants TsAma(R)-1 and TsAma(R)-8 was accompanied by an alteration in the level of alpha-amanitin resistance of the RNA polymerase II activities in the revertant cells. Together these data provide convincing evidence that TS mutations in RNA polymerase II can be coselected with alpha-amanitin resistance.

Ribonucleic acid polymerase mutant of Escherichia coli defective in flagella formation

Escherichia coli K-12 mutants that are resistant to bacteriophage chi, defective in motility, and unable to grow at high temperature (42 degrees C) were isolated from among those selected for rifampin resistance at low temperature (30 degrees C) after mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine. Genetic analysis of one such mutant indicated the presence of two mutations that probably affect the beta subunit of ribonucleic acid (RNA) polymerase: one (rif) causing rifampin resistance and the other (Ts-74) conferring resistance to phage chi (and loss of motility) and temperature sensitivity for growth. Observations with an electron microscope revealed that the number of flagella per mutant cell was significantly reduced, suggesting that the Ts-74 mutation somehow affected flagella formation at the permissive temperature. When a mutant culture was transferred from 30 to 42 degrees C, deoxyribonucleic acid synthesis accelerated normally, but RNA or protein synthesis was enhanced relatively little. The rate of synthesis of beta and beta' subunits of RNA polymerase was low even at 30 degrees C and was further reduced at 42 degrees C, in contrast to the parental wild-type strain. Expression of the lactose and other sugar fermentation operons, as well as lysogenization with phage lambda, occurred normally at 30 degrees C, suggesting that the mutation does not cause general shut-off of gene expression regulated by cyclic adenosine 3',5'-monophosphate.

Temperature-sensitive ribonucleic acid polymerase mutant of Salmonella typhimurium with a defect in the beta' subunit

Localized mutagenes of Salmonella typhimurium followed by a [3H]uridine enrichment procedure yielded a temperature-sensitive strain with a mutation in the rpo region of the chromosome. Ribonucleic acid (RNA) polymerase (EC 2.7.7.6; nucleoside triphosphate: RNA nucleotidyltransferase) purified from this mutant was considerably less active at the nonpermissive temperature than wild-type enzyme. Furthermore, the enzyme from this mutant, unlike RNA polymerase of previously isolated temperature-sensitive mutants, was as thermostable as wild-type enzyme when preincubated at 50 degrees C. Subunit reconstitution experiments have shown that the temperature sensitivity is caused by an alteration in the beta' subunit of the enzyme.

Rifampicin-induced protein synthesis: A pre-requisite for increased expression of the beta beta' operon in Escherichia coli

In a rifS/rifR heterodiploid strain of E. coli, a 4 minute pulse of rifampicin can induce a prolonged (greater than 60 min) increase in the rate of synthesis of the RNA polymerase subunits, beta and beta'. The application of a constraint on the fidelity of protein synthesis during, but not after, the rifampicin pulse partially arrests the development of this capacity for subunit synthesis. I discuss the implications of these findings in relation to the control of the BB' operon in E. coli.

The influence of mutations upon the synthesis of RNA polymerase subunits in Escherichia coli cells

The influence of mutations in structural genes of beta and beta subunits of RNA polymerase upon the synthesis of these subunits in E. coli cells have been investigated. An amber-mutation ts22 in the beta subunit gene decreases the intracellular concentration of this subunit and the rate of its synthesis. At the same time the concentration and the rate of beta subunit synthesis is increased. These suggest the compensatory activation of the RNA polymerase operon that takes place under the conditions of shortage of one of the subunits. Reversions as well as more effective suppression of ts22 amber mutation, achieved by streptomycin addition, substitution of su2 by sul, or by specific mutations, result in a rise of beta and drop of beta subunit concentration and synthesis in ts22 mutant. TsX missense-mutation in the beta subunit gene alters the properties of the enzyme increasing, at the same time, the concentration and the rate of synthesis of both beta and beta subunits, particularly at a nonpermissive temperature. This points to an inversely proportional relationship between the rate of synthesis of RNA polymerase subunits and the total intracellular activity of the enzyme. Extra subunits are rapidly degraded in ts22 and tsX mutants. The whole complex of our data and those of others suggest that the regulation of the synthesis of RNA polymerase subunits is accomplished by interaction of a negative and a positive mechanisms of regulation which include not only activators and repressors but the enzyme itself as well.

In vivo distribution of ribonucleic acid polymerase between cytoplasm and nucleoid in Escherichia coli

Three forms of ribonucleic acid polymerase can be distinguished in exponentially growing Escherichia coli cells: (i) active, (ii) inactive, inside the nucleoid, and (iii) inactive, free in the cytoplasm.

Temperature-sensitive RNA polymerase mutants with altered subunit synthesis and degradation

A temperature-sensitive mutant having a lethal mutation in the gene for the beta subunit of RNA polymerase (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6) exhibits an apparent 2- to 3-fold decrease in the rates of both beta and beta' subunit synthesis at the non-permissive temperature, relative to total protein. In contrast, a temperature-sensitive mutant with a lethal mutation in the gene encoding beta' has a 5- to 6-fold increase in the rates of beta and beta' synthesis at 42 degrees. These beta and beta' mutants also exhibit rapid degradation of both subunits at the high temperature.