In vitro synthesis of polypeptides encoded by the long terminal repeat region of mouse mammary tumour virus DNA

Abstract

The integrated proviral DNA of RNA tumour viruses is bounded by long terminal repeat (LTR) segments of several hundred base pairs which result from duplication of sequences from both the 5′ and 3′ ends of the viral genome RNA^1–3. Recently, DNA sequence analysis of the LTRs of several retroviruses has indicated that this region of the provirus may contain regulatory functions for viral RNA transcription ^4–8. In this connection, mouse mammary tumour virus (MuMTV) is of particular interest in that transcription of the viral RNA can be specifically modulated by glucocorticoid hormones, suggesting that the site of steroid action may be contained within the LTR⁹. Moreover, the LTR of MuMTV is unusually long, extending for about 1,300 base pairs. We now present data which suggest that, in contrast with other retroviruses, the MuMTV LTR includes approximately 1,000 nucleotides in an open reading frame, capable of encoding a series of polypeptides which are quite distinct from any of the known structural components of MuMTV virions. These proteins can be expressed in vitro from cloned, recombinant DNA in which the MuMTV LTR has been inserted into the ampicillin-resistance gene of a bacterial plasmid. Tryptic peptide mapping has confirmed that these products are identical to the analogous series of proteins identified by in vitro translation of polyadenylated fragments of MuMTV genome RNA¹⁰.