Terminal direct repeats in a retrovirus-like repeated mouse gene family

The mouse genome contains multiple copies of a dispersed gene family known individually as VL30 genes¹ which are thought to be associated with retroviruses. The copies consist of closely related 5.2-kilobase DNA sequences flanked by unrelated cellular DNA, and although no sequence homology has been found between the genes and the exogenous or endogenous retroviruses so far tested, the 30S RNA transcripts expressed by the genes are efficiently packaged into virions, recovered from infected cells^2-4 and transmitted to other cells by pseudo-type infection⁵. Stimulated by recent reports^6-10 of the similarity between retroviruses and transposons (from which retroviruses may have evolved⁶), and in particular by the recognition that both types of genetic elements are characterized by a large terminal repeat (LTR), we set out to determine whether the VL30 genes are also distinguished by this property. Using cloned DNAs from a mouse gene library and heteroduplex analysis, we have now found that the VL30 genes do indeed carry terminal direct repeats 400 base pairs long.