High-resolution organization of mouse centromeric and pericentromeric DNA

We studied the organization of mouse satellite 3 and 4 (MS3 and MS4) in comparison with major (MaSat) and minor (MiSat) DNA sequences, located in the centromeric and pericentromeric regions of mouse telocentric chromosomes by fiber-FISH. The centromeric region consists of a small block of MiSat and MS3 followed by a pericentromeric block of MaSat with MS4. Inside the block of the long-range cluster, MaSat repeats intermingle mostly with MS4, while MiSat intermingle with MS3. The distribution of GC-rich satellite DNA fragments is less strict than that of AT-rich fragments; it is possible to find MS3 fragments in the MaSat array and MS4 fragments in the MiSat array. The methylation pattern does not fully correspond to one of the four families of satellite DNA (satDNA). In each satDNA fragment only part of the DNA is methylated. MS3 and MS4 are heavily methylated being GC-rich. Pericentomeric satellite DNA fragments are more methylated than centromeric ones. Among the four families of satDNA MS4 is the most methylated while MiSat is methylated only to a minimal extent. Estimation of the average fragment length and average distance between fragments shows that the range of the probes used does not cover the whole centromeric region. The existence of unknown sequences in the mouse centromere is likely.