Activation of the transcriptase in purified reovirus by chymotrypsin requires facilitating conditions in the digestion mix. In the presence of facilitation the virions are uncoated to cores. Without facilitation, uncoating is blocked at an intermediate stage, with transcriptase latent. Two independent types of facilitation have been discovered. The first type is mediated by K⁺, Rb⁺ or Cs⁺ ions. A second type, described here, is mediated by suitably high concentrations of virus in the digestion mix and requires collisions between intermediate subviral particles for its occurrence. The virus-concentration-mediated process is reversibly inhibited by high levels of active chymotrypsin.

Intermediate subviral particles (ISVP) derived from reovirus represent a simple model system for the switch-on of transcriptase function. In such particles the endogenous transcriptase is present in a switched-off form, one step removed from the switched-on state. Switch-on of transcriptase function is an active process in this system and can be triggered by K+ions. A variety of agents which affect gene expression in cells were tested for an effect on switch-on in ISVP. Marked effects on switch-on in ISVP were observed with a diverse group of test agents, including DMSO and other solvents, BUdR, TdR, caffeine, theophylline, and temperature. The correlation in response between ISVP and cells suggests that the ISVP system may be useful as a model for studying the biochemical mechanisms underlying the perturbative effects of such agents on gene expression in cells.