Isolation of the mouse mammary tumor virus: chemical and morphological studies.

Abstract

The course of purification of the mammary tumor virus from milk of infective Columbia RIII strain mice by methods involving velocity gradient (zone) centrifugation in Ficoll, a chemically inert, high molecular weight, synthetic polysaccharide, is described. On a dry weight basis, an approximate 1,000-fold enrichment of virus was achieved. A fraction consisting of what appeared to be incomplete virus was separated in a zone above the main viral particle zone in the gradient tube. The viral particles had an average lipid content of 27 percent and a ribonucleic acid (RN A) content of 0.8 percent on a dry weight basis. Experiments pointed to the essential single-stranded character of the RNA, the mononucleotide composition of which, in moles percent, was: uridine monophosphate, 28.9; guanosine monophosphate, 30.2; cytidine monophosphate, 21.6; adenosine monophosphate, 19.3. The value of 3.7 × 106 daltons was determined for the average RNA content of the virions. In certain morphological features and in aspects of their maturation, largely relating to the arrangement of the nucleic acid-containing structure within the particles, they differed significantly from the myxoviruses, a group which they otherwise resemble in general morphology and gross chemical composition. Substantial tumorigenicity for the mammary gland was found to reside in viral particle preparations, purified by isopyenic gradient centrifugation in rubidium chloride and gradient electrophoresis, methods previously described—the purity of which was attested by electron microscopic examination.