In vitro interactions between virus and enhancer.

Cell-free supernatant fluids of permissive host cells contained an enhancer for the m+ variant of echovirus 6. The enhancer activity remained in supernatant fractions after centrifugation at 110,000 x g for 4 hr. Under in vitro conditions, a complex between enhancer and virus formed rapidly. The complex was not dissociated by dilution and was sedimented by centrifugation. The effect of enhancer on the distribution of virus in CsCl density gradients was determined. In the absence of enhancer, virus was recovered in one peak at 1.33 g/ml. Incubation of enhancer with virus before centrifugation resulted in the formation of two peaks of infectivity at 1.33 g/ml and 1.24 g/ml. The heavier virus peak represented 43% of the recovered infectivity and could be enhanced. The light virus peak contained 26% of the infectivity and could not be enhanced. The density of enhancer was 1.20 g/ml and approximated the density of the light virus population.