Primate and rodent genomes are populated with hundreds of thousands copies of 
Alu and B1 elements dispersed by retroposition, i.e., by genomic reintegration 
of their reverse transcribed RNAs. These, as well as primate BC200 and rodent 
4.5S RNAs, are ancestrally related to the terminal portions of 7SL RNA sequence. 
The secondary structure of 7SL RNA (an integral component of the signal 
recognition particle) is conserved from prokaryotes to distant eukaryotic 
species. Yet only in primates and rodents did this molecule give rise to 
retroposing Alu and B1 RNAs and to apparently functional BC200 and 4.5S RNAs. To 
understand this transition and the underlying molecular events, we examined, by 
comparative analysis, the evolution of RNA structure in this family of molecules 
derived from 7SL RNA. RNA sequences of different simian (mostly human) and 
prosimian Alu subfamilies as well as rodent B1 repeats were derived from their 
genomic consensus sequences taken from the literature and our unpublished 
results (prosimian and New World Monkey). RNA secondary structures were 
determined by enzymatic studies (new data on 4.5S RNA are presented) and/or 
energy minimization analyses followed by phylogenetic comparison. Although, with 
the exception of 4.5S RNA, all 7SL-derived RNA species maintain the cruciform 
structure of their progenitor, the details of 7SL RNA folding domains are 
modified to a different extent in various RNA groups. Novel motifs found in 
retropositionally active RNAs are conserved among Alu and B1 subfamilies in 
different genomes. In RNAs that do not proliferate by retroposition these motifs 
are modified further. This indicates structural adaptation of 7SL-like RNA 
molecules to novel functions, presumably mediated by specific interactions with 
proteins; these functions were either useful for the host or served the selfish 
propagation of RNA templates within the host genome.