Plant Molecular Biology 36: 717–724, 1998.
1998 Kluwer Academic Publishers. Printed in Belgium.
Molecular characterization of a single mitochondria-associated
double-stranded RNA in the green alga Bryopsis
Ryuichi Koga, Toshiyuki Fukuhara
and Takeshi Nitta
Laboratory of Molecular and Cellular Biology, Faculty of Agriculture, Tokyo University of Agriculture and
Technology, Fuchu, Tokyo 183, Japan (
author for correspondence)
Received 5 June 1997; accepted in revised form 23 October 1997
Key words: Bryopsis, double-stranded RNA, frameshift translation, mitochondrial plasmid, RNA-dependent RNA
Mitochondria from the green alga Bryopsis sp. very often contained a 4.5 kb double-stranded RNA (dsRNA) at a
deﬁned level. Complementary DNA 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. Consensus sequence for
RNA-dependent RNA polymerase was found within the longer ORF (2472 bp) of the dsRNA. The overlapping
52 bp of the ORFs in different reading frames is suggestive of the occurrence of a
1 ribosomal frameshift in
the mitochondrial translation system. The observed simple genetic structures suggest that the algal mitochondrial
dsRNA might be deﬁcient in a gene for movement from cell to cell in host plants and, hence, has a plasmid-like
nature that is distinct from that of infectious plant viruses. The nature and origin of the endogenous dsRNAs of
various sizes and their relationships are discussed.
Indigenous dsRNAs of various sizes have been found
in large populations of symptomless plants from algae
to higher plants [7, 24]. The dsRNAs have important
common properties that are closer to those of plasmids
cell division and their propagation is regulated strin-
gently, with maintenance of deﬁned low levels, in cells
sequence of their ability to replicate efﬁciently in cells
of their host plants as do naturally occurring plasmids.
Such dsRNAs have been found not only in the plant
kingdom but also in a variety of fungi , protozoa
 and insects .
The nucleotide sequence data reported will appear in the DDBJ,
EMBL and GenBank Nucleotide Sequence Databases under the
accession number D88669.
ous dsRNAs appear to be bound somehow to capsid
proteins in their host eukaryotic cells. In particular,
smaller dsRNAs from many higher plants are tightly
associated with protein, forming isometric particles
that are stable in chloroform or butanol and were
deﬁned as cryptoviruses (family, Partitiviridae) .
The particles are 30–40 nm in diameter, typically con-
and a single major capsid polypeptide of about 55 kDa
acterized as unique viruses in view of the absence of
reviews, see [1, 34]). By contrast, indigenous plant
dsRNAs larger than those of cryptoviruses have not
been classiﬁed into any plant viral groups; these lar-
ger dsRNAs are not associated with distinct virus-like
particles, and the largest ones of more than 10 kb are