Isolation and characterization of three rat U3 RNA pseudogenes colinear with U3 RNA.

Three different 15-kilobase rat genomic clones that contained sequences colinear with U3 RNA were isolated. These inserts hybridized only to U3 RNA in a mixture of total cellular 4-8 S RNA labeled in vivo which showed that genes or pseudogenes for most other small RNAs were absent in these U3 DNA clones. DNA sequence analysis showed that the three subcloned genes contained full-length U3-coding sequences but each had sequence variations, insertions, and/or deletions when compared to rat U3A or U3B RNA. Two of these pseudogenes contained poly(A) sequences on the 3'-end and were flanked by 6-15-nucleotide long direct repeats. None of the three clones was transcribed when injected into Xenopus oocyte nuclei. One clone was a template for a small RNA slightly larger than U3 RNA, but this transcript was not related to the U3 RNA sequences. The structural features of two of these three U3 DNAs are supportive of the hypothesis that some pseudogenes arose from RNA-mediated DNA synthesis and insertion into the genome at random sites (Van Arsdell, S. W., Denison, R.A., Bernstein, L.B., Weiner, A.M., Manser, T., and Gesteland, R.F. (1981) Cell 26, 11-20). This is the first instance where full-length, colinear, U3 RNA pseudogenes have been isolated and characterized.