Organization of the R family and other interspersed repetitive DNA sequences in the mouse genome.

Six R sequences have been described as members of a new family of dispersed repetitive DNA in the mouse genome (Gebhard et al., 1982). Several sequenced regions were extended in the 5' direction (R1, R2 and R6) and two new sequences were determined (R7 and R8). On the basis of our sequence and blot hybridization data it is concluded that the R sequence are adjacent to the so-called small Bam family (Fanning, 1982), which in turn runs into the MIF sequence part (Brown & Piechaczyk, 1983) of the large Bam sequences (Meunier-Rotival et al., 1982). In one of our clones a sequence of 1290 base-pairs comprises MIF, Bam and R sequences in a contiguous arrangement which seems to be characteristic of the long repeat unit of the mouse genome. Several repeat units were found to be truncated within their Bam or R sequence parts. Evidence is also reported for transposition events involving R sequences; for instance of one R sequence (R1) into another (R7). Two R sequences (R1 and R4) have apparently been transposed together with part of the adjacent Bam sequences. Truncation and transposition events may also explain the imbalance of copy numbers within the large repeat unit (25,000 to 50,000 for the small Bam sequences and 100,000 for the R sequences). The spreading of R sequences and other interspersed DNA sequences within the mouse genome may have occurred by transposition events on the DNA level and/or by transcription, retrotranscription and insertion processes.