Cell cycle-dependent alterations of the two types of ribonucleases H in L5178y cells

Renaturase and ribonuclease H: a novel mechanism that influences transcript displacement by RNA polymerase II in vitro

I have previously reported an activity in HeLa cells which facilitates transcript displacement by purified mammalian RNA polymerase II in vitro. I have shown that this activity copurifies with one of two separable ribonuclease (RNase) H activities in HeLa cells. The RNase H activity in question has characteristics similar to those reported for RNase H2b from calf thymus. RNase H proteins purified from several other sources including Escherichia coli also show renaturase activity. When the renaturase/RNase H protein is present during transcription by purified RNA polymerase II, transcripts are truncated close to the 5' end, and the remainder of the transcript is displaced normally from its template by the polymerase. Since RNA polymerase II dependent transcripts in vivo normally require the presence of the 5'-triphosphate terminus for capping, the in vivo significance of RNase H as a renaturase factor is presently not understood. However, the in vitro action of renaturase/RNase H suggests that the mechanism of this reaction may involve R-loop displacement after formation of a short single-stranded region of DNA on the template strand following hydrolysis of a hybrid transcript oligonucleotide by RNase H.

Structure and expression of the dnaQ mutator and the RNase H genes of Escherichia coli: overlap of the promoter regions

A 1.6-kilobase-pair DNA fragment derived from the Escherichia coli chromosome was analyzed by Tn3 transposon insertion and deletion mapping to locate a mutator gene, dnaQ (mutD), and the rnh gene that codes for RNase H. When a strong promoter, PL of lambda phage, was placed at the right- and left-side of the cloned DNA fragment, the dnaQ protein and RNase H, respectively were overproduced. These results suggested that the two genes are transcribed in opposite directions and that their promoters are located in a narrow region between the genes. Nucleotide sequence analysis confirmed this and further revealed that transcriptional and translational initiation signals for the two genes overlap. From the sequence data it was deduced that the dnaQ protein and RNase H consist of 243 and 155 triplets and have molecular weights of 27,500 and 17,500, respectively. dnaQ81 amber mutant showed two codon alterations, CAG(glutamine-195) leads to TAG(amber) and ACA(threonine-193) leads to ATA(isoleucine). The dnaQ-lacZ and the rnh-lacZ fused genes were constructed and hybrid proteins with beta-galactosidase activity were produced. From beta-galactosidase levels it was estimated that the promoter for dnaQ is 5 times more active than that for rnh.

Arabinosyl nucleosides. XXXII. Mechanism of inhibition of L5178y mouse lymphoma cells by 9-beta-D-arabinofuranosylguanine

The mechanism of the growth-inhibitory action of 9-beta-D-arabinofuranosylguanine (ara-G) on mouse lymphoma cells (L5178y) at the level of nucleic acid synthesis was studied. Ara-G inhibits the cell proliferation at an ED50 concentration of 12.3 microM. The cells die because of unbalanced growth. At cytostatic concentrations ara-G inhibits the incorporation rate of dThd into DNA to a much higher degree compared to those of the precursors into RNA or proteins. Ara-G has no effect on the induction of DNA polymerase alpha occurring at the beginning of the S-phase; this compound exerts its inhibitory activity during the S-phase. Ara-G is intracellularly phosphorylated to ara-GTP. It was demonstrated that ara-G is incorporated into DNA but not into RNA; one molecule of ara-G is incorporated per 7050 molecules of deoxyguanosine. The ara-GMP moieties, incorporated into DNA of L5178y cells are not removed during a culture period of 7.7 doubling steps; compared to the controls no reduction of the viability and growth rate was observed.