Nature of transient inhibition of deoxyribonucleic acid synthesis in HeLa cells by parainfluenza virus 1 (Sendai)

Adsorption of ultraviolet-inactivated Sendai virus, at high or low multiplicity, to HeLa cells caused a transient increased incorporation of (3)H-thymidine into the cellular deoxyribonucleic acid (DNA). In HeLa cells synchronized by a double-thymidine block, this increased incorporation of thymidine during the S phase lasted from about 30 to 90 min after virus adsorption. The observations that the kinetics of accumulation of radioactive thymidine in the nucleotide pool did not differ in control and in the virus-treated cells and that the (32)P incorporation into the DNA of the virus-treated cells was inhibited at the same time indicate that the augmented incorporation of (3)H-thymidine into DNA results from a transient block in the endogenous pathway of thymidine synthesis. Chromatographic analysis of the nucleotide pool of the virus-treated cells labeled with (14)C-formate indicates that methylation of deoxyuridine monophosphate to thymidine monophosphate is inhibited. It is suggested that the inhibition is caused by a block of either the thymidilate synthetase or some step in the tetrahydrofolate cycle.

Cell fusion by various strains of Newcastle disease virus and their virulence

Some properties of eight strains of Newcastle disease virus (cell-fusing ability, hemolysin, heat stability of hemagglutinin or of hemolysin) do not correlate with virulence of these strains.