Examination of the protein components of highly purified polyoma virions by electrophoretic separation in SDS-polyacrylamide gels has revealed the presence of three previously undetected protein bands. Two of these, designated P3a and P4a, were not found in lysates of infected cells and may represent modified products of their respective doublet partners, P3b and P4b. The third, P6a, along with P5, P6b, and P7 are electrophoretically indistinguishable from protein bands present in the nuclear fraction of both infected and uninfected cells, and represent the virion counterparts of the host cell histones. Comparisons of the major nonhistone proteins, P2, P3, and P4, by tryptic peptide analyses have shown that P3 and P4 are closely related, but that P2 appears to be independently coded.
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