VL30 : a mouse retrovirus-like family of repetitive DNA elements
Mouse and rat cells encode an abundant 30S RNA which shares many structural properties with retroviral genomic RNA. This VL30 RNA can be efficiently packaged into retrovirus particles. Mouse cells recently infected with a MuLV (VL30) pseudotype were shown to contain full length, reverse-transcribed DNA copies of both RNA species. VL30 DNA could also be synthesized in quantity using the endogenous reverse transcriptase activity of detergent-disrupted MuLV (VL30) particles. This DNA was found to be identical to that produced in vivo. Several 4.6-4.9kbp molecular clones (NVL clones) of VL30 cDNA were obtained. The retrovirus-like LTRs of each clone displayed a moderate restriction enzyme site heterogeneity, but NVL unique sequence was identical in each case. Southern blotting experiments using NVL probes showed that (a) most of the 100-200 NIH-3T3 DNA mouse VL30 elements were organized into provirus-like structures with a high degree of sequence conservation, and (b) the majority of these elements were hypermethylated and transcriptionally inactlve, whereas an expressed NVL-like sub-class could account for no more than 5% of mouse VL30 genes. NVL-related sequences in rodent DNAs other than the mouse were markedly less abundant and showed a greater sequence divergence. This was in contrast to MuLV-related sequences whose copy number and homology to a cloned MuLV probe decreased more gradually with phylogenetic distance from the mouse. Sub-genomic NVL probes showed that two rodent species had each conserved a different block of NVL-like sequence. These data indicate that each family has exhibited a different rate of sequence divergence during rodent evolution. Finally, a MuLV (VL30)-infected rat fibroblast line was shown to have received 1-2 copies per cell of a transcriptionally active NVL-like element. This suggests the possibility that evolution of each rodent VL30 family has been influenced by retrovirus-mediated transmission across the species barrier.