Plant Genome I Conference
Town & Country Conference Center, San Diego, CA, November, 1992.
PG-I: A VARIANT TRANSCRIPT IS DETECTED IN A STERILE LINE OF VICIA
FABA
A VARIANT TRANSCRIPT IS DETECTED IN A STERILE LINE OF VICIA
FABA.
D. Cheepers and M. Bawdry, Unite de Biochimie Physiologique,
Universite Catholique de Louvain, Place Croix du Sud, 2-20,
B-1348 Louvain-la-Neuve, Belgium.
The higher plant mitochondrial genome is characterized by
its large size and also by a complex molecular organization
involving sequence rearrangements generated through recombination
processes. We have been studying the possible effect of these
rearrangements on transcription by comparing a fertile and a
sterile line of Vicia faba characterized by a different
restriction pattern. Thus we used individual clones from a
mitochondrial library as probes in Northern blot experiments. We
found a 3.2-kb DNA fragment (EF3) which hybridized to eight
transcripts ranging from 800 to 3600 nucleotides, two of them
being 150 nt larger in the fertile than in the sterile cytoplasm.
S1 nuclease mapping and cDNA cloning indicated that the EF3
fragment contains two exons separated by a class II intron but no
difference could be detected in the transcript segments assigned
to this fragment. Sequence analysis indicated an identical
primary structure between the two cytoplasms except for 8 point
mutations. Sequence comparison with data banks indicated that
these two exons correspond to the last two exons of the nad5
gene. The nad5 gene consist of 5 exons (a+b, c, d+e) dispersed
in three different regions of the mitochondrial genome. Tne
production of the mature RNA requires two cis- and two
trans-splicing steps. Using PCR experiments on cDNA, we isolated
part of exons a and b. We used them in Northern blot experiments
but no variant transcripts were detected. A oligonucleotide
complementary to exon c (which is only 22 bp long) was used to
select a genomic fragment carrying exon c. The latter was then
hybridized to mtRNA from F and S lines and variants transcripts
were again revealed. Variants transcripts were thus correlated
with exons c, d and e only and not with exons a and b. Further
investigations are necessary to localize the origin of the
variant RNAs.
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