Characterization of JHMV variants isolated from rat brain and cultured neural cells after wild type JHMV infection

Evidence for variable rates of recombination in the MHV genome

Mouse hepatitis virus has been shown to undergo RNA recombination at high frequency during mixed infection. Temperature-sensitive mutants were isolated using 5-fluorouracil and 5-azacytidine as mutagen. Six RNA+ mutants that reside within a single complementation group mapping within the S glycoprotein gene of MHV-A59 were isolated which did not cause syncytium at the restrictive temperature. Using standard genetic techniques, a recombination map was established that indicated that these mutants mapped into two distinct domains designated F1 and F2. These genetic domains may correspond to mutations mapping within the S1 and S2 glycoproteins, respectively, and suggest that both the S1 and S2 domains are important in eliciting the fusogenic activity of the S glycoprotein gene. In addition, assuming that most distal ts alleles map roughly 4.0 kb apart, a recombination frequency of 1% per 575-676 bp was predicted through the S glycoprotein gene. Interestingly, this represents a threefold increase in the recombination frequency as compared to rates predicted through the polymerase region. The increase in the recombination rate was probably not due to recombination events resulting in large deletions or insertions (greater than 50 bp), but rather was probably due to a combination of homologous and nonhomologous recombination. A variety of explanations could account for the increased rates of recombination in the S gene.

On the membrane cytopathology of mouse hepatitis virus infection as probed by a semi-permeable translation-inhibiting drug

Previous studies of the membrane fusion process have permitted the characterization of membrane permeability changes concomitant with MHV-induced cytopathology. One indication of membrane permeability in MHV-infected cells is their sensitivity to translation inhibition by the normally impermeable amino-glycoside, hygromycin B (Macintyre, G., Wong, F. and Anderson, R. (1989) J. Gen. Virol. 70, 763-768). In the present study, we examine the hygromycin B sensitivity of acutely infected mouse fibroblast L-2 cell and macrophage cultures as well as persistently infected mouse fibroblast LM-K cell cultures. The results suggest that membrane permeability alterations (as indicated by hygromycin B sensitivity) are a common feature of these MHV infections. Hygromycin B "cured" persistently infected LM-K cells as indicated by the absence of detectable virus antigen by immunofluorescence and by the absence of infectious virus even after removal of the drug or co-cultivation with untreated L-2 cells. The results argue against the maintenance of MHV infection by a mechanism involving latently or non-cytolytically infected cells. We conclude therefore that at least one mechanism for MHV persistence depends on virus propagation by cytolytic infection of a small, dynamically changing, fraction of the total cells present in culture.