Cold sensitive mutant of Escherichia coli with altered RNA polymerase

On the early evolution of RNA polymerase

The lines of evidence suggesting that RNA preceded double-stranded DNA as an informational macromolecule are briefly reviewed. RNA polymerase is hypothesized to have been one of the earliest proteins to appear. It is argued that an important vestige of the original enzyme is found in the contemporary eubacterial beta' subunit of DNA-dependent RNA polymerase and its homologues among the archaebacterial and eukaryotic enzymes. The evidence that supports a catalytic role in replicase activity of this polypeptide is reviewed. It is suggested that several characteristics of the Escherichia coli transcriptional apparatus are relatively recent evolutionary developments. The phylogenetic importance of the eubacterial beta' subunit from RNA polymerase and its homologues is emphasized, because it allows the study of the evolutionary relationships of the major cellular lines (eubacteria, archaebacteria, and eukaryotes) as well as of some viral lineages.

Cold-sensitive growth of simian virus 40 in semipermissive variants of CV1 cells

Two cell clones were isolated from the simian line CV1, permissive for simian virus 40 (SV40), by selection at low temperature with the tsA239 mutant of SV40. These clones exhibited cold-sensitive semipermissivity to both SV40 virions and SV40 DNA. On the basis of virus yields, their resistance to viral DNA was increased approximately 15 times over that of CV1 cells when the incubation temperature was lowered from 38.5 to 33.5 degrees C. A further 30- to 40-fold resistance increase was exhibited at both temperatures upon infection with SV40 virions. Partial characterization of these clones indicated that the cold sensitivity affected an early function in viral growth, between viral uncoating and the appearance of T-antigen positivity, with a burst-size decrease in all cells at the restricted temperature. This conditional defect appeared to be superimposed upon a temperature-independent uncoating defect, presumably carried in a CV1 subpopulation from which the two clones were ultimately selected.

Cold-sensitive mutations in beta and beta' subunit gene affecting the interaction of RNA polymerase with promoters

RNA polymerases with a cold-sensitive activity were purified from seven mutants of Escherichia coli. Subunit reconstitution experiments have shown that RNA polymerases from three mutants (Rpob262, RpoB264, and RpoB265) owed their cold sensitivity to alterations in the beta subunit. Three mutants (RpoC3, RpoC263, and RpoC267) were shown to be defective in the beta' subunit and one (RpoBC266) in both beta and beta' subunits. Two mutations (rpoC3 and rpoC263) reduced the level of RNA polymerase reconstitution. RNA polymerases from RpoC3 and RpoBC266 mutants are defective in RNA chain elongation at 6 degree C, while all the other mutants are defective in RNA polymerase-promoter interaction. most mutant RNA polymerases differ from the wild-type enzyme in transcription selectivity. The results obtained in this study indicate that both beta and beta' subunits are involved in RNA chain elongation and promoter binding and selection.

A cold-sensitive beta subunit mutant RNA polymerase from Escherichia coli with defects in promoter opening in vitro

A cold-sensitive mutation in the rpoB gene for the RNA polymerase beta subunit increasing the temperature of promoter opening on T2 phage DNA was obtained in Escherichia coli. The mutation also affects the stages preceding promoter opening by increasing the dissociation rate of RNA polymerase--DNA closed complexes. The affinity of RNA polymerase to T2 and lambda DNA is differentially changed by the mutation. The relative efficiency of transcription of these two templates is also changed. These results suggest a participation of the RNA polymerase beta subunit in the interaction with promoters.

Isolation of cold-sensitive Chinese hamster cells

Six cold-sensitive variants have been isolated from Chinese hamster ovary cells by the BUdR-visible light selection technique. The properties of one of these lines have been studied in detail. This line stops dividing immediately after a shift from 39 degrees C to 33 degrees C though its doubling time at 39 degrees C is only slightly longer than that of wild-type cells. The rates of DNA and protein synthesis are severely reduced at 33 degrees C, but the rate of RNA synthesis is not significantly different from wild-type cells. This line may be defective in protein synthesis, but the results of sedimentation analysis indicate that it probably has normal ribosomal subunit assembly.