Plant Genome I Conference
Town & Country Conference Center, San Diego, CA, November, 1992.
PG-I: 36pg1
RNA EDITING IN PLANT MITOCHONDRIA: THE FATE OF THE a-PHOSPHATE
OF CMP RESIDUES.
R. Michael Mulligan and V.K. Rajasekhar, Department of
Developmental and Cell Biology, University of California, Irvine,
CA 92717
RNA editing in higher plant mitochondria results in the
post-transcriptional conversion of C to U residues. The
mechanisms by which this transition could occur are: 1) de- or
trans-amination of the amide at C-4 of cytosine; 2)
transglycosylation of the ribosyl residue at C-l', resulting in
base replacement; and 3) deletion and insertion of the edited
nucleotide. Each mechanism results in retention or loss of
different portions of the nucleotide of the RNA; a deletion and
insertion mechanism would be expected to break the phosphodiester
backbone and result in exchange of the entire nucleotide. In
order to discriminate among these mechanisms, plant mitochondria
were supplied with a- 32P-CTP to radiolabel CMP residues in the
newly synthesized transcripts. Radiolabeled RNA was extracted,
purified and digested with nuclease Pl to convert the residues to
nucleoside monophosphates. The composition of the resultant NMPs
derived from the radiolabeled transcripts was analyzed by one-
and two-dimensional TLC. Autoradiography of these products
indicated the abundant presence of CMP residues in the
radiolabeled RNA, as well as a small fraction of radioactivity
that co-migrates with UMP on two dimensional TLC. The amount of
radioactivity in UMP increased over the course of the incubation.
These results suggest that RNA editing occurred in isolated
intact mitochondria. Recovery of radiolabeled UMP residues after
incorporation as CMP residues indicates that phosphodiester
exchange does not occur during editing process. These results
are consistent with either (1) deamination or (2)
transglycosylation mechanisms for RNA editing, but are
inconsistent with (3) a deletion and insertion mechanism that
would be expected to exchange the I phosphate.
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