Infection with phage T2 can overcome the thymine requirement of a mutant of Escherichia coli.' Specifically, infection induces a thymidylate synthetase that is

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SUMMARYAnalysis of six temperature-sensitive (ts) mutants of Newcastle disease virus (NDV) representing each of six complementation groups by both SDSpolyacrylamide gel electrophoresis (SDS-PAGE) and two-dimensional gel electrophoresis revealed that in only one mutant was there an alteration in the isoelectric point of a protein. This altered protein was the nucleocapsid-associated protein, NAP. In addition, the mobility of the haemagglutinin-neuraminidase protein, HN, was decreased on non-reduced SDS-PAGE in this mutant.All independent ts + clones derived from this mutant had normal NAP but HN protein migrated at the decreased rate. Haemagglutinating activity of wild-type (ts +) and ts virions was equally thermostable. Wild-type and ts + clones derived from this ts mutant were RNA (+) at both permissive and non-permissive temperatures, whereas the ts mutant was RNA(-) at the non-permissive temperature. This ts mutant appears to be a double mutant in both HN and NAP genes, the latter only being a temperature-sensitive lesion which affects virus-directed RNA synthesis.