Abundant authentic MMTV-Env production from a recombinant provirus lacking the major LTR promoter

As for all retroviruses, the env mRNA is thought to be a singly spliced product of the full-length transcript from the P1 promoter in the MMTV provirus. However, we show that envelope proteins can be produced in an inducible manner in the absence of the P1 promoter from an otherwise complete provirus. Furthermore, we demonstrate in both reporter assays and the proviral context that the R region is necessary for protein production in transiently transfected cells and in a number of independent, stably transfected cell clones. Using 5′ RACE, we show that a sequence within the R region functions as a TATA less initiator. The most distal part of the 5′ LTR (first 804 bases of the U3 region) is required for the activity of the R-initiator element only when the provirus is integrated. Transfection with a full-length proviral DNA carrying a deletion of P1 in the 5′ LTR resulted in the establishment of stable cell clones able to produce Env in a dexamethasone-dependent manner but not infectious virions. We therefore conclude that in the absence of P1, R can drive transcription of the spliced env mRNA but not genomic viral RNA.