DNA complementary to rabbit globin mRNA made by E. coli polymerase I

Starvation-associated mutation in E. coli strains with and without reverse transcriptase

Starvation-associated mutation to Tyr+ in E. coli WU3610 is identical to that in an isogenic derivative lacking reverse transcriptase encoded by a retron phage.

Retroelements in bacteria

A peculiar type of satellite DNA, called msDNA, has been discovered in myxobacteria and some natural isolates of E. coli. These molecules are characterized by the presence of single-stranded DNA branching out from an internal guanosine residue of an RNA molecule by a unique 2',5'-phosphodiester linkage. Reverse transcriptase is required for the synthesis of msDNA. The discovery of retroelements in bacterial populations raises many intriguing questions concerning the evolutionary origin of reverse transcriptase, the function and the biosynthesis of msDNA, and the nature of the mechanisms generating the extensive diversity found in msDNA and reverse transcriptase genes among different bacterial strains.

The intracellular content of dihydrofolate reductase: possibilities for control and implications for chemotherapy

Intracellular levels of DHFR can be modulated by mechanisms other than gene amplification. We found that MTX itself has an effect and the important features of this mechanism are as follows: (a) Sub-saturating doses of MTX induce intracellular DHFR activity by increasing DHFR synthesis; (b) The time-dependent effect seems quite specific for DHFR and is reversible (7); (c) Elevated DHFR synthesis is accompanied by disproportionate increases in DHFR mRNA; (d) The time scale for maximum induction is appreciably longer than the cell generation time. We suggest that part of the control involved is translational and we postulate that DHFR may regulate its own biosynthesis through feedback mechanisms. It is conceivable that the induction phenomenon could affect the clinical efficacy of MTX-therapy in some instances.

Synthesis of compositionally unique DNA by terminal deoxynucleotidyl transferase

Studies on the composition and characterization of DNA product(s) synthesized by calf thymus terminal deoxynucleotidyl transferase were performed using homopolymeric single-stranded, calf thymus double-stranded, and native DNA resident in calf thymus chromatin preparations as priming DNA species. Synthesis was carried out using equimolar concentrations of all four deoxynucleoside triphosphates as substrates and Mg2+ or Mn2+ as an effective divalent cation. Irrespective of the nature of the priming DNA or the divalent cation, the DNA product contained 60-70% dGMP residues, 10-15% each of the two pyrimidine residues, and 5-10% dAMP residues. The product synthesized using chromatin DNA as initiator was predominantly single-stranded and its synthesis was resistant to actinomycin D. The predilection of terminal deoxynucleotidyl transferase to synthesize dGMP-rich products on natural or homopolymeric DNA primers suggests that such products may represent biologically important recognition signal sequences.

Template-specific requirements for DNA synthesis by the Mason-Pfizer monkey virus DNA polymerase: unique aspects

The biochemical properties of DNA polymerase purified from Mason-Pfizer monkey virus were studied, with respect to synthetic and natural template-primer utilization. Thes studies revealed the following new information about the Mason-Pfizer monkey virus enzyme: (a) Mason-Pfizer monkey virus polymerase was found to prefer template: primer molar nucleotide ratios of 2.5-5: 1 for optimal rates of synthesis with poly(C) .(dG)12-18 as template-primer. (b) Poly(A)-directed synthesis was stimulated by the addition of low concentrations of inorganic phosphate to the reaction mixture. (c) Poly(2' -O-methyl-cytidylate), poly(rCm), was the only template studied for which Mn2+ proved the preferred divalent cation. Combinations of divalent cations stimulated rather than inhibited poly(rCm)-directed poly(dG) synthesis by the Mason-Pfizer monkey virus enzyme. (d) Heteropolymeric regions of rabbit globin mRNA and avian myeloblastosis virus 70 S RNA could be copied by the Mason-Pfizer monkey virus polymerase with oligo(dT), oligo(U) or in the case of avian myeloblastosis virus RNA, endogenous primers. In all such studies, Mg2+ was the preferred divalent cation and a distinct preference for the DNA primer in the reverse transcription of natural RNAs was observed. These new findings necessitated comparative studies with the DNA polymerases from Rauscher murine leukemia virus and murine mammary tumor virus, as representative type C and type B retroviruses. Although the Mason-Pfizer monkey virus enzyme was found to share some properties in common with both type C and type B mammalian viral enzymes, certain of the above properties rendered it unique among the polymerases examined.

Reverse transcriptase: correlation of zinc content with activity

Evidence is presented that DNA polymerase of avian myeloblastosis virus has an obligatory zinc requirement for activity. Previous studies indicate that the purified polymerase contains zinc in a stoichiometry of about 1 g-atom/mole. We now find that the enzyme-bound zinc is exchangeable with radioactive (65)Zn; after isoelectric focusing, the radioactive (65)Zn is coincident with polymerase activity. Dialysis of the (65)Zn-labeled polymerase against the chelator, 1,10-phenanthroline, results in a progressive loss of radioactive (65)Zn and polymerase activity. Thereupon, incubation of the inactivated enzyme with Zn(2+) fully restores activity. Thus, the DNA polymerase present in an oncogenic RNA virus, like animal DNA polymerases, can be rigorously classified as a zinc metalloenzyme. DNA polymerase of avian myeloblastosis virus is inactivated by 1,10-phenanthroline at a much faster rate than the bacterial and animal DNA polymerases that have been tested. It may, therefore, be possible to inactivate selectively DNA polymerases from animal tumor viruses by brief exposure to appropriate metal chelators.

Conditions for using DNA polymerase I as an RNA-dependent DNA polymerase

Conditions are described for using Escherichia coli DNA polymerase I for synthesizing complementary DNA copies of natural RNA molecules, which are suitable for use in hybridization experiments. The molar ratio of enzyme to template is critical; below a certain level, synthesis is not observed. Hybrids formed with the complementary DNA are of comparable specificity and stability to those formed with complementary DNAs synthesized by viral RNA-directed DNA polymerase. Synthesis of dA-dT polymers, a common occurrence with this enzyme, can be eliminated by including distamycin in the reaction mixture.