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

Abstract

The genome of Mus musculus contains multiple copies (500-1000) of DNA sequences related to the 35S RNA of intracisternal type A particles (IAP). Using labeled IAP RNA as a probe in blot-hybridization experiments, a characteristic elctrophoretic pattern of reactive fragments generated by restriction endonuclease cleavage of mouse DNA was identified. From the genomic blots, a composite restriction map for a 6.5- to 7-kilobase (kb) DNA region containing sequences homologous to the IAP RNA was constructed. 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 plasmic pBR322 from a 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 6 different library isolates containing 6.5-7-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. Mouse DNA may contain other dispersed sequence elements related to but smaller than the genetic unit defined above.