The 5S ribosomal RNA sequences have been determined for the rhodoplast of the red alga Porphyra umbilicalis and the chloroplast of the conifer Juniperus media. The 5S RNA sequence of the Vicia faba chloroplast is corrected with respect to a previous report. A survey of the known sequences and secondary structures of 5S RNAs from plastids and cyanobacteria shows a close structural similarity between all 5S RNAs from land plant chloroplasts. The algal plastid 5S RNAs on the other hand show much more structural diversity and have certain structural features in common with bacterial 5S RNAs. A dendrogram constructed from the aligned sequences by a clustering algorithm points to a common ancestor for the present-living cyanobacteria and the land plant plastids. However, the algal plastids branch off at an early stage within the plastid-cyanobacteria cluster, before the divergence between cyanobacteria and land plant chloroplasts. This evolutionary picture points to the occurrence of multiple endosymbiotic events, with the ancestors of the present algal plastids already established as photosynthetic endosymbionts at a time when the ancestors of the present land plant chloroplasts were still free-living cells.

The corrected nucleotide sequences of the cytoplasmic 5 S ribosomal RNAs of 6 angiosperms are reported. The previously reported sequences, reconstructed from oligonucleotide catalogs, had been suspected for some time to contain errors. The corrected seqeuences fit in a universal J-helii model for 5 S RNA secondary structure. The model involves the existence of a dynamic ~uilibrium between slightly different base-pairing schemes in two areas of the structure. Reconstruction of a phylogenetic tree from over 200 known 5 S RNA sequences yields a general picture of biological evolution. It confirms the tripartite descent of living species -eubacteria, archaebacteria, eukaryotes -and the endosymbiotic origin of plant mitochondria and chloroplasts.5 S rRNA Secondary ~t~ct~re