Human SRP RNA and E. coli 4.5S RNA contain a highly homologous structural domain.

At present the most powerful tool available for determining the secondary structure of an RNA is the phylogenetic approach (Noller, 1984). This has been the primary method used to derive the secondary structure of the RNA component of the signal recognition particle (SRP), 7SL RNA (herein referred to as SRP RNA; Ullu et al., 1982). This analysis was carried out using the sequences of the human, frog, and fly SRP RNAs (Gundelfinger et al., 1984; Ullu and Tschudi, 1984; Zwieb, 1985; Zwieb and Ullu, 1986), all of which are functionally interchangeable in chimeric particles reconstituted in vitro (Walter and Blobel, 1983). More recently, RNAs from the yeasts Schizosaccharomyces pombe (Brennwald et al., 1988; Poritz et al., 1988; Ribes et al., 1988) and Yarrowia lipolytica (Poritz et al., 1988) and the archaebacterium Halobacterium halobium (Moritz et al., 1985) have been identified and suggested to be homologs of SRP RNA. The wide evolutionary diversity represented in this collection of sequences has allowed us to refine the phylogenetic description of the SRP RNA secondary structure and has revealed a unique and highly conserved structural domain. Surprisingly, we find this domain in the 4.5s RNA of Escherichia coli (Hsu et al., 1984) and in the small cytoplasmic RNA (scRNA) of Bacillus subtilis (Struck et al., 1988).