SUMMARYA thin sectioning procedure was used to detect and enumerate oncornavirus particles in tumour-cell culture fluids, tumour homogenates, mouse blood plasma, mouse and human milk specimens, and density gradient fractions of these specimens. Oncornaviruses studied included mouse mammary tumour, murine sarcoma, ESP-I, and RD-II 4 viruses. In this procedure particles were sedimented on to small membrane filter discs in the ultracentrifuge using inexpensive commercially available adapters and tubes. Particles in cross sections of the disc were counted and their total number determined by relating the effective surface area of a field to the surface area of the entire membrane disc. Particles may be reliably identified and counted in preparations containing predominately cellular debris. Linear dose response plots were obtained in serial dilution experiments utilising vaccinia virus, adenovirus type 2, and murine sarcoma virus, demonstrating the reliability of the procedure and its wide applicability. The counts obtained for adenovirus and vaccinia virus preparations were comparable to counts obtained for the same preparations in other laboratories by established methods. Statistically reliable counts have been obtained using sample vol. of o.oi ml or less.
Two types of genomic, high-molecular-weight RNA species were found in Soehner-Dmochowski murine sarcoma virions released from virus-induced rat tumor cells grown in tissue culture. The type of RNA species observed depended on the length of exposure of the tumor cells to radioactive precursor. Early RNA of virions labeled up to 4 h with radioactive uridine had a sedimentation coefficient of 50
S
, and late RNA of virions labeled for 24 h had a sedimentation coefficient of 58
S
. Thermal transitions of early and late RNA indicated a difference in the configuration or structure of these two types of RNA. The late RNA may represent either a different configurational state of the early RNA or an aggregate molecule of two early RNA components joined together. Heat dissociation revealed that the major subunit of both RNA types was a 28
S
species, which was not susceptible to degradation by the addition of micrococcal nuclease to virions. A transitional, intermediate RNA species with a sedimentation coefficient of 37 to 40
S
was detected when early RNA was dissociated by dimethyl sulfoxide or heat at temperatures suboptimal for complete conversion. No free RNA subunit components were detected in virions harvested at intervals as short as 30 s or 5 min. A model for the assembly of genomic RNA from 28
S
RNA subunits is proposed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.