We have shown previously that pseudogenes for the small nuclear RNAs U1, U2, and U3 are dispersed in the human genome and are much more abundant than the snRNA genes themselves, a finding that contrasts sharply with the general paucity of pseudogenes in other multigene families. The reason for the abundance of snRNA pseudogenes became apparent when we noticed that the snRNA sequence in many pseudogenes was exactly flanked by short direct repeats of 16 to 19 base pairs, and we have argued that such pseudogenes must be created by the insertion of snRNA information into staggered breaks at new chromosomal loci in germline cells (Van Arsdell et al., 1981). Subsequently, by comparing the DNA sequence of additional U1 genes and pseudogenes, we have been able to show that some U1 pseudogenes which lack flanking direct repeats were created by insertion of U1 snRNA information into blunt breaks at new chromosomal loci. We argued that the invading snRNA information must be derived from a cDNA copy of the snRNA, and in support of this hypothesis we have recently found that in the presence of avian myeloblastosis virus (AMV) reverse transcriptase, purified U3 snRNA is capable of self-primed reverse transcription in which the 3 feet end of the snRNA primes the synthesis of a specific cDNA corresponding to the 5 feet third of the snRNA sequence; moreover, although purified U2 snRNA is incapable of such reverse transcription, purified small nuclear ribonucleoprotein particles (snRNPs) can be reverse transcribed without added primer to yield a specific cDNA spanning the 5 feet quarter of the snRNA sequence. For both U2 snRNPs and purified U3 snRNA, the cDNA approximately coincides with the characteristically truncated snRNA sequence found between the direct repeats in the pseudogenes. We propose (1) to further characterize the self-primed reverse transcription of U2 snRNPs and U3 snRNA, and to determine whether cellular DNA polymerases Beta and Gamma can substitute for the AMV enzyme; (2) to examine the ability of structurally defined single-stranded cDNAs and covalent RNA:DNA hybrids to integrate into the host chromosome after microinjection into cultured mammalian cells and the fertilized eggs of Xenopus; and (3) to continue our studies of snRNA gene organization, with a special emphasis on chromosomal mapping of the U1 and U2 gene families.
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