Intracisternal A-particle genes: identification in the genome of Mus musculus and comparison of multiple isolates from a mouse gene library.

The genome of Mus musculus contains multiple copies (500 -1000) of DNA sequences related to the 35S RNA of intracisternal type A particles (IAPs). Using labeled IAP RNA as a probe in blot-hybridization experiments, we have identified a characteristic electrophoretic pattern of reactive fragments generated by restriction endonuclease cleavage of mouse DNA. From the genomic blots, we deduced a composite restriction map for a 6.5- to 7-kilobase (kb) DNA region containing sequences homologous to the IAP RNA. Units of this type appeared to be interspersed without obvious regularity in nonhomologous flanking regions. A 5.2-kb segment of this unit was inserted directly into plasmid pBR322 from HindIII/EcoRI digest of mouse DNA. The fragment was cloned and then labeled by nick-translation and used to scan a mouse embryo gene library (average 16-kb inserts in lambda Charon 4A); 1% of the library samples hybridized, confirming the extensive reiteration of IAP genetic units. Among six different library isolates containing 6.5- to 7-kb IAP units, some restriction sites were highly conserved whereas others varied in both occurrence and position. Despite this variation, heteroduplexes between the individual isolates showed continuous IAP homology regions of 7 kb. No flanking region homologies were seen in this limited sample. Some evidence suggests that mouse DNA may contain other dispersed sequence elements related to but smaller than the genetic unit defined above.