Chlorella viruses prevent multiple infections by depolarizing the host membrane

Previous experiments established that when the unicellular green alga Chlorella NC64A is inoculated with two viruses, usually only one virus replicates in a single cell. That is, the viruses mutually exclude one another. In the current study, we explore the possibility that virus-induced host membrane depolarization, at least partially caused by a virus-encoded K+ channel (Kcv), is involved in this mutual exclusion. Two chlorella viruses, PBCV-1 and NY-2A, were chosen for the study because (i) they can be distinguished by real-time PCR and (ii) they exhibit differential sensitivity to Cs+, a well-known K+ channel blocker. PBCV-1-induced host membrane depolarization, Kcv channel activity and plaque formation are only slightly affected by Cs+, whereas all three NY-2A-induced events are strongly inhibited by Cs+. The addition of one virus 5-15 min before the other results primarily in replication of the first virus. However, if virus NY-2A-induced membrane depolarization of the host is blocked by Cs+, PBCV-1 is not excluded. We conclude that virus-induced membrane depolarization is at least partially responsible for the exclusion phenomenon.