MOLECULAR CHARACTERIZATION OF A SINGLE MITOCHONDRIA-ASSOCIATED DOUBLE-STRANDED-RNA IN THE GREEN-ALGA BRYOPSIS

Mitochondria from the green alga Bryopsis sp, very often contained a 4 .5 kb double-stranded RNA (dsRNA) at a defined level. Complementary DN A probes derived from the mitochondrial dsRNA hybridized with none of the algal chloroplast dsRNAs of 1.7 to 2.2 kb, but did hybridize with a similar-sized dsRNA among several dsRNAs from the mitochondria of B. maxima. Sequence analysis of the mitochondrial dsRNA from Bryopsis sp . revealed only two large, overlapping, open reading frames (ORFs) on one strand if UGA was taken as a non-termination codon, suggesting the independent phylogenetic evolution of the mitochondrial dsRNA. Consen sus sequence for RNA-dependent RNA polymerase was found within the lon ger ORF (2472 bp) of the dsRNA. The overlapping 52 bp of the ORFs in d ifferent reading frames is suggestive of the occurrence of a -1 riboso mal frameshift in the mitochondrial translation system. The observed s imple genetic structures suggest that the algal mitochondrial dsRNA mi ght be deficient in a gene for movement from cell to cell in host plan ts and, hence, has a plasmid-like nature that is distinct from that of infectious plant viruses. The nature and origin of the endogenous dsR NAs of various sizes and their relationships are discussed.