Selection of a mutant S1 gene during reovirus persistent infection of L cells: role in maintenance of the persistent state.

LR-7 cells, variant L cells derived from a type 3 reovirus persistently infected (p.i.) carrier culture (R. Ahmed, W. M. Canning, R. S. Kauffman, A. H. Sharpe, J. V. Hallum, and B. N. Fields, Cell 25, 325-332, 1983) were used to define the viral genes critical for maintenance of the persistent state. A cloned viral isolate (L/C virus) derived from the p.i. culture replicated normally in LR-7 cells, while wild-type (wt) viruses of the three reovirus serotypes replicated less efficiently. To identify the viral gene(s) permitting enhanced replication of L/C virus in LR-7 cells, viral reassortants were prepared by mixed infection of L cells with L/C virus and type 1 wt. Study of the one-step growth curves and final yields of large numbers of reassortants in both L cells and LR-7 cells revealed that the presence of the S1 gene from L/C virus was critical for normal viral replication in LR-7 cells. However, this phenotype was suppressed by the simultaneous presence in reassortants of both the M2 and S4 genes from the type 1 wt parent. The critical change in the S1 gene occurred by passage 13 (63 days) after initiation of the carrier culture. Although multiple mutations are present in the viral population from p.i. cultures, certain specific mutations can be identified as critical for maintenance of the persistent state.