L cells lacking thymidine kinase (TK) activity (Ltk-cells) have been stably transformed to a TK-positive phenotype by infection with ultraviolet-irradiated herpes simplex virus (HSV-UV). The highest frequency of the Ltkto Ltk+ transformation observed in these experiments was approximately 10-3, whereas no measurable transformation was observed (less than 10-8) in the absence of HSV-UV infection. Cell lines of HSV-transformed Ltk+ cell lines contain 7 to 24 times as much TK activity as do the parental Ltk-cells, and they have been maintained in culture for a period exceeding 8 months. The kinetics of thermal inactivation of the TK activity derived from an Ltk+ HSV-transformed cell line and the TK activity from Ltkc cells lytically infected with infectious HSV are similar. Both of these TK activities are much more thermolabile than the TK activity present in wild-type L cells. A mutant strain of HSV which does not induce TK activity during lytic infection does not cause the Ltkto Ltk+ transformation. These data suggest that either an HSV TK gene has been transferred to Ltk-cells or that an HSV gene product has caused the expression of a previously repressed cellular enzyme.

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The multiplication of a new, ether-sensitive, ribonucleic acid virus, 229E, isolated from the human respiratory tract, has been studied in cultures of WI-38 human diploid cells. In thin sections of these cells examined with the electron microscope, particles appeared in vesicles in the cytoplasm of cells at a time corresponding to the initial increase in infectious virus. Antigen was also detected in the cytoplasm of cells by the immunofluorescent technique. Extracellular particles of similar morphology were prominent soon after. These events preceded a detectable cytopathic effect. Later, an electron-dense particle appeared within vacuoles in the cytoplasm but was never found extracellularly. Its role in virus development is not known. Complement-fixing antigen developed along with the increase in infectious virus.

The presence of adenosine triphosphate, guanosine triphosphate, cytosine triphosphate, or uridine triphosphate reduced the rate of inactivation of vaccinia when heated at 50 C. The virus-associated nucleoside triphosphate phosphohydrolases (adenosine triphosphatase, guanosine triphosphatase, cytosine triphosphatase, and uridine triphosphatase) and ribonucleic acid polymerase were also protected from heat inactivation by these compounds. These obervations are best explained by postulating that ribonucleoside triphosphates bind to enzymes in the virus particle, and that these enzyme-substrate complexes are more resistant to thermal denaturation than are the enzymes without their substrates. The kinetics of heat inactivation of the vaccinia ATP phosphohydrolase activity is biphasic, suggesting that there are two proteins in the vaccinia particle that have this enzyme activity but they have different kinetics of heat inactivation. Any of the vaccinia-associated nucleotide phosphohydrolase activities are protected from heat inactivation by the presence of any one of the respective nucleoside triphosphates. This observation suggests that there is a single enzymatic site in vaccinia that is able to react with any ribonucleoside triphosphate. Wallis, Yang, and Melnick (14) reported that the rate of heat inactivation of vaccinia is reduced by the presence of 2.0 M NaCl or 50%