Extreme heterogeneity of minor satellite repeat arrays in inbred strains of mice.

Abstract

The centromeric regions of mammalian chromosomes contain tandemly repeated satellite sequences that are late replicating and display characteristics of constitutive heterochromatin. Satellite sequences in Mus musculus are relatively AT rich and are methylated at their CpG dinucleotides, indicative of the lack or low level of transcriptional activity. Two classes of centromeric repetitive sequences have been characterized in mouse, the major (gamma) and minor satellite sequences (Hastie 1989). The mouse minor satellite may constitute up to 0.5% of its genome, and the haploid copy number has been estimated to be at least 50,000 (Pietras et al. 1983). Minor satellite sequences are found at the primary constriction on the autosomes and the X Chromosome (Chr), but are absent from the Y Chr. These sequences are thought to be organized in largely uninterrupted blocks of tandem repeats (Kipling et al. 1991). Cytologically, minor satellite sequences appear to be localized at the kinetochore (Wong and Rattner 1988). A surprising aspect of centromeric satellite repeats is that they are not conserved across species. Closely related species of Mus differ in the amount and type of satellite sequence present in their centromeric heterochromatin. Mus spretus has relatively little major satellite sequence but contains minor satellite, whereas Mus caroli has proportionately more major satellite but lacks the minor satellite sequences, and Mus musculus has appreciable amounts of both classes of satellite sequences (Wong et al. 1990). Within the species Mus musculus, short-range restriction fragment length polymorphisms of minor satellite have been demonstrated among several strains of inbred mice (Siracusa 1985). In addition, Kipling and associates (1994) have observed long-range polymorphisms in minor satellite between two widely used inbred lines, C57BL/6 and DBA/2. We have extended these studies by examining the long-range organization of the regions containing minor satellite sequences in 18 inbred strains of mice. Southern analysis of genomic DNA fragments resolved by pulsed field gels using oligonucleotide probes for the minor satellite sequence reveals large-scale DNA polymorphisms between all the strains examined. In Fig. 1, HaelII digests of genomic DNA from one wildderived and 17 inbred mouse strains are probed with an oligonucleotide complementary to minor satellite, #49-GTAACTCACTCATCTAATATG, derived from position 49 through 29 antiparallel to the consensus sequence for minor satellite (Wong and Rattner 1988). When the blot in Fig. 1 is reprobed with another minor satellite-derived oligonucleotide, #60-ACATTCGTTGGAAACGGGA, from positions 60 through 78 of the consensus sequence, which contains the putative binding site for the centromeric antigen CENP-B, the same hybridization pattern is seen. These oligonucleotide probes do not hybridize to rat (Rattus norvegicus) genomic DNA under the conditions used (results not shown), indicating that the two oligonucleotides are binding spe-