Native ribonucleoprotein is an efficient transcriptional complex of avian myeloblastosis virus

Abstract

A native ribonucleoprotein (RNP) complex of avian myeloblastosis virus was prepared under conditions that gave optimal cDNA synthesis. The complex was an autonomous transcriptional unit capable of synthesizing DNA complementary to the RNA virus genome in the absence of exogenous reverse transcriptase (RNA-dependent DNA nucleotidyltransferase), genomic RNA, and primer. The RNA of the RNP complex cannot be translated in an in vitro cell-free translational system. The RNP contains intact viral RNA, the two subunits of the reverse transcriptase (beta and alpha), the p32 polypeptide resulting from the cleavage of the beta subunit into the alpha subunit, and p12. The principal polypeptide constituent of the RNP complex is the highly basic protein p12, which occurs at a molar ratio of 40:1 in relation to the beta subunit of the polymerase. When examined by the electron microscope, the RNP complex appears similar to the beaded structure of chromatin fiber. A significant portion of these molecules are circular, with headlike structures attached. The circular nature of the proviral DNA and the ability of the RNP complex to generate large intact cDNA copies from the natural primer end suggest that the 5' and 3' ends of the viral RNA are in proximity when in the RNP complex.