Retrovirus insertion inactivates mouse α 1(I) collagen gene by blocking initiation of transcription

Abstract

Mov13 mice carry a single Moloney murine leukaemia virus (M–MuLV) proviral copy in the first intron of the α1(I) collagen gene. Virus insertion interferes with the synthesis of stable α1(I) collagen messenger RNA and causes a recessive lethal mutation^1,2. The virus insertion has induced changes of the methylation pattern³ as well as the chromatin conformation in the mutated gene. Specifically, a DNase-hypersensitive site which is associated with active transcription of the wild-type collagen gene is not present in the mutant allele⁴. The block of collagen expression could be caused by virus-induced instability of collagen mRNA or by impaired initiation of transcription. To distinguish between these possibilities, we have compared the activity of the α1(I)collagen gene promoter in cell lines derived from wild-type and Mov 13 embryos by nuclear run-on transcription experiments and S₁ mapping of nuclear RNA. We show here that initiation of transcription of the mutant gene is reduced 20–100-fold. This indicates that the virus-induced change of chromatin structure in the promoter region of the mutant gene prevents RNA polymerase from binding to its DNA template. Our results are consistent with the notion that the promoter-associated DNase-hypersensitive site is a prerequisite for rather than a consequence of gene activity.