Novel repetitive sequence families showing size and frequency polymorphism in the genomes of mice

A middle repetitive sequence, PR1, originally found in mouse rDNA appeared as satellite-like bands when EcoRI and BglII digests of genomic DNA were subjected to Southern blot hybridization using PR1 as probe. The copy number and sizes of PR1-related satellite-like bands, designated as PR1 families, differed remarkably among the subspecies and laboratory strains of mice when the EcoRI digests of genomic DNAs were compared. These bands were not detected in rat and human DNAs. A unit of PR1 sequence was determined by examining cloned EcoRI 3.5 kb (kb, 10(3) bases) fragment and 6.6 kb rDNA by cross-hybridization and sequence analysis: 3.5 kb and 6.6 kb DNAs are composed of homologous PR1 regions and the flanking non-homologous sequences. The results indicate that amplification of different sequences containing PR1 has occurred in different subspecies and strains of mice, and that the segments of satellite-like bands are likely to have been created by recombination of the PR1 sequence with other DNA segments before amplification. The chromosomal distribution of the 3.5 kb PR1 family was studied by back-crossing the female F1 between BALB/c and DDD/1 to male DDD/1. The segregation data strongly suggest that most, if not all, of this family are located on a single chromosome. The stability of these PR1 families in the genomes of cultured cells of a given strain was also examined. An extra band homologous to PR1 appeared in their genomes, but was not detected in other tissues, indicating that some PR1 families may change even during cell propagation.