Emv-13 (Akv-3): a noninducible endogenous ecotropic provirus of AKR/J mice

All AKR/J mice carry at least three endogenous ecotropic viral loci which have been designated Emv-11 (Akv-1), Emv-13 (Akv-3), and Emv-14 (Akv-4) (Jenkins et al., J. Virol. 43:26-36, 1982.) Using two independent AKR/J-derived sets of recombinant inbred mouse strains, AKXL (AKR/J x C57L/J) and AKXD (AKR/J x DBA/2J), as well as the HP/EiTy strain (an Emv-13-carrying inbred strain partially related to AKR/J mice) (Taylor et al., J. Virol. 23:106-109, 1977), we have examined the association of these endogenous viral loci with virus expression. Strains which transmit Emv-11 or Emv-14 or both were found to produce virus spontaneously, whereas strains that transmit Emv-13 alone were negative for virus expression. Restriction endonuclease digestion and hybridization with an ecotropic virus-specific hybridization probe of DNAs from strains which transmit only Emv-13 yielded enzyme cleavage patterns identical to those observed with DNAs from strains transmitting Emv-11 or Emv-14 or both. These findings indicate the absence of any gross rearrangement of Emv-13 proviral sequences. Cell cultures derived from recombinant inbred strains that carry only Emv-13 failed to express detectable infectious virus, viral proteins, or cytoplasmic ecotropic virus-specific RNA even after treatment with 5-iodo-2-deoxyuridine or 5-azacytidine, an inhibitor of DNA methylation. Our results indicate that a mechanism(s) other than methylation of Emv-13 proviral DNA is responsible for inhibition of Emv-13 expression.