[Mapping of 2 cloned DNA fragments containing repetitive mouse genome regions, actively transcribing into pre-mRNA]

Sequence organization of two cloned mouse genomic DNA fragments which are efficiently transcribed into pre-mRNA was studied. In both cases, the arrangement of sequences hybridizing to polysomal mRNA (or cDNA), double-stranded RNA (dsRNA-B) isolated from pre-mRNA and oligo (dT), was established. In one fragment, the regions transcribed are represented by three different repetitive mRNA (or cDNA) hybridizing sequences interspersed into the AT-rich tract, the type B-2 sequence and the unique DNA sequence which also transcribed into mRNA. Nuclear pre-mRNA of 28S is hybridized to three repetitive mRNA hybridizing regions. The significance of such a complex array of repeated sequences inside eukaryotic transcription units and their possible role in mRNA processing and splicing is discussed.