Virions of the WSN strain of influenza A0 virus have been found to contain three polypeptides designated P1, P2, and P3, with molecular weights of 96,000, 87,000, and 85,000, respectively. In studies of the time course of synthesis of viral proteins in chick embryo fibroblasts (CEF), these three polypeptides and viral polypeptides NP, M, and NS have been detected within 1 hr of infection. Experiments in which cycloheximide was added at the time of infection to restrict viral RNA synthesis to primary transcription for various times after infection, and then protein synthesis examined after removal of the drug, have indicated that all of the viral polypeptides can be translated from mRNA produced by primary transcription (with the possible exception of the neuraminidase polypeptide which could not be detected with certainty in the cells under the conditions used). Although these results suggest that transcripts of the entire genome are produced early in infection and by primary transcription, there is control of viral protein synthesis. This was demonstrated by findings, such as increasing rates of synthesis of the M polypeptide relative to the other proteins after longer periods of primary transcription, and a relatively small amount of polypeptide HA synthesized immediately after removal of cycloheximide. The results obtained provide evidence for control of protein synthesis at the level of transcription, but do not exclude or establish control of translation as well. In CEF cells no increase in the rate of synthesis of the three P polypeptides was detected, and their rates of synthesis declined within 4-6 hr, whereas the synthesis of the other polypeptides was amplified within 11 1 2 hr after infection or removal of cycloheximide. In contrast, in MDBK cells which produce higher titers of infectious virus and fewer incomplete particles than CEF cells, amplification of the synthesis of P polypeptides, as well as of the other polypeptides, was observed, suggesting a host-dependent effect on the control of the synthesis of these influenza virus proteins which are thought to be involved in viral RNA synthesis.
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