Further characterization of virus-like 30S (VL30) RNA of mice: initiation of reverse transcription and intracellular synthesis.

We have studied the virus-like 30S (VL30) RNA sequences of mice. Previous work has shown that these sequences are coded in the mouse genome, expressed in some normal cells and released as pseudotypic particles from cells producing murine C-type retroviruses. VL30 sequences have some similarities to standard retrovirus RNA, but differences also exist. To further assess the similarities and differences, several aspects of VL30-specific metabolism were investigated. We studied the initiation of VL30-specific DNA synthesis during an endogenous reverse transcriptase reaction. Short initial VL30-specific cDNA transcripts were covalently attached to RNA as measured by equilibrium banding in caesium sulphate density gradients. Therefore, reverse transcription of VL30-specific cDNA is initiated by an RNA primer. The intracellular synthesis of VL30 RNA was investigated by pulse labelling uninfected JLS-V9 cells with 3H-uridine. Hybridization of the pulse-labelled nuclear RNA indicated that the major VL30-specific RNA evident after a 15 min label was the same size as the mature VL30 RNA. Thus, VL30 RNA is apparently not synthesized via a higher mol. wt. precursor. Both of these results demonstrate similarity of VL30 RNA sequences to standard retroviruses. One unique feature of VL30 RNA was detected. JLS-V9 cells contained both the monomeric VL30 RNA and a hydrogen-bonded 38S form which yielded the monomer when denatured. This contrasts with standard murine leukaemia virus which is only found as a monomer within cells.