Human genes and pseudogenes for the 7SL RNA component of signal recognition particle

Of the several hundred 7SL RNA-like sequences that are dispersed in human DNA, no more than four are likely to represent genes for 7SL RNA; the majority are 7SL pseudogenes which appear to result from the reverse flow of genetic information from 7SL RNA back into genomic DNA. We present the sequence of five 7SL pseudogenes displaying an unprecedented diversity of structures. All are truncated copies of 7SL RNA, but the site of truncation can occur at either the 5' end, the 3' end or at both ends of the RNA sequence. We suggest that such diverse 7SL pseudogenes are generated by different but related pathways. In particular, we argue that two of the loci are secondary 7SL pseudogenes which derive from RNA polymerase III transcripts of primary (preexisting) 7SL pseudogenes. We also report the isolation and characterisation of a human genomic clone carrying two linked 7SL RNA coding regions, 7L30.1 and 7L30.2. The 7L30.2 locus differs by several single base changes from the known human 7SL RNA sequences and does not appear to be expressed at a detectable level in HeLa cells. The 7L30.1 locus is an authentic 7SL RNA gene encoding one of the three sequence variants of human 7SL RNA.