Patterns of genomic distribution and sequence heterogeneity of a murine "retrovirus-like" multigene family

The mouse genome contains over 100 copies of a dispersed gene family known as "virus-like" genes encoding 30S RNA (VL30). Although they do not share nucleotide sequence homology with known retroviruses, these genetic elements are distinguished by several "retrovirus-like" features, notably, the capacity of the 30S RNA transcripts of these genes to be encapsidated by c-type virions and the transmissibility of VL30 information to other cells via pseudovirion infection. Using VL30 DNA units, cloned from the BALB/c mouse embryonic gene library, we have recently shown that VL30 DNA units share basic structural features with retrovirus proviruses. To shed light on the relatedness of VL30 information to endogenous proviruses and possibly other genetic elements, we extended our previous studies concerning genomic distribution patterns of VL30 elements and patterns of sequence heterogeneity among VL30 units. The following observations were made: (i) VL30 units were distributed among different mouse chromosomes; (ii) distribution patterns of VL30 units markedly differed among mouse strains; (iii) there was constancy of VL30 restriction patterns in different tissues; (iv) a high degree of sequence divergence existed among different VL30 units cloned from the same embryo; and (v) VL30 units were heterogeneous with respect to the state of DNA methylation. The results are discussed in terms of the possible modes of evolution of this multigene family.