Secondary structure of 7SK and 7-2 small RNAs. Possible origin of some 7SK pseudogenes from cDNA formed through self-priming by 7SK RNA.

Pseudogenes having homology to small RNAs, like 7SL, 7SK, 6S, 4.5S, U1, U2, and U3 RNAs, are abundant and dispersed in the genomes of higher eukaryotes [reviewed in Weiner et al. (1986) Annu. Rev. Biochem. 55, 631-661]. To understand better the possible origin of these pseudogenes, we studied the abilities of cytoplasmic 7SL, 7SK, and nucleolar 7-2 RNAs to self-prime and result in the synthesis of cDNAs. When rat 7SK RNA was used as substrate, a 294-nucleotide-long cDNA was synthesized in vitro by reverse transcriptase, indicating that the 3' end of 7SK RNA can act in a self-priming manner to generate 7SK cDNA. When 7-2 RNA was used as a substrate, a cDNA of approximately 235 nucleotides was observed; 7SL RNA did not act as a self-primer. Earlier studies have shown that DNAs homologous to 7SK RNA are represented by a moderately reiterated family in the mammalian genomes and many of these sequences were found to be truncated 7SK pseudogenes [Murphy et al. (1984) J. Mol. Biol. 177, 575-590]. In this study, one 7SK clone from the rat genome was characterized by sequencing. This clone contained 243 base pairs homologous to the 5' end of 7SK RNA, and was flanked by direct repeats. These data suggest that, as previously proposed for some U3 pseudogenes [Bernstein et al. (1983) Cell 32, 461-472], one mechanism for the generation of truncated 7SK pseudogenes may be the integration of self-primed reverse transcripts of 7SK RNA at random genomic sites.