Disruption of the putative splice acceptor site for SIVmac239Vif reveals tight control of SIV splicing and impaired replication in Vif non-permissive cells

Vif is dispensable for simian immunodeficiency virus (SIV) replication in some cells, termed permissive (i.e., CEM-SS), but not in others, termed non-permissive (i.e., H9, CEMx174, and peripheral blood lymphocytes). Non-permissive cells express the RNA editing enzyme, APOBEC3G. To determine whether vif mRNA could be alternatively spliced, a mutation altering the putative vif splice acceptor site (SA1) was introduced into SIVmac239 (SIVΔvif-SA). Despite three consensus splice acceptor sites nearby SA1, SIVΔvif-SA did not efficiently generate alternatively spliced vif mRNA. SIVΔvif-SA was growth attenuated in CEMx174 and H9 cells but not in CEM-SS cells. Following SIVΔvif-SA, but not SIVmac239, infection in either H9 or CEMx174 cells viral cDNA contained numerous G to A mutations; no such differences were observed in CEM-SS cells. This pattern is consistent with mutations generated by APOBEC3G in the absence of Vif. Therefore, efficient splicing of SIV vif mRNA is tightly controlled and requires the SA1 site.