Plant Genome II Conference
Town & Country Conference Center, San Diego, CA, January, 1994.
PG-II: ZEA MAYS MITOCHONDRIAL GENOME: ORGANIZATION FUNCTION AND
EVOLUTION
ZEA MAYS MITOCHONDRIAL GENOME: ORGANIZATION FUNCTION AND
EVOLUTION
Christiane M.-R. Fauron, Marc Casper and Yan Gao. Eccles Genetics
Building, University of Utah, Salt Lake City, UT 84112
Higher plant mitochondrial genomes (mt DNAs) are large (200
kb to 2500 kb) and variable in size between and within species.
Within the genus Zea mays, the mtDNA of different lines also
varies in size and organization. We have identified, mapped and
compared two different normal (e.g. male fertile) maize cytotypes
named NA and NB, a cytoplasmic male sterile: cmsT, and we are now
characterizing various teosinte mtDNAs. All the genetic
complexity can be arranged onto an hypothetical circular DNA
molecule called the master chromosome. The master chromosome for
cmsT, NB and NA are 540kb, 570kb and 700kb respectively. These
master chromosomes contain various sets of repeated sequences
which undergo homologous recombination giving rise to isomeric
forms (direct repeats) or subgenomic circular DNA molecules
(inverted repeats) whose size are dependent upon the location and
orientation of the repeats within the master chromosomes. The
high frequency of recombination is perhaps the most unique
feature of higher plant mtDNA. Comparative structural studies
between these genomes reveal a rather different organization from
each other. Each genome can be divided into regions whose
boundaries are defined at positions where the restriction map
fails to be identical between the genomes that are being
compared. Forty one breakpoints have been mapped in NA when
compared with NB and cmsT. NA, NB and cmsT have 49lkbp of
identical sequences (but not necessarily in the same orientation)
and only two sets of identical repeats. NA is an interesting
evolutionary link as it contains sequences present in cmsT but
not NB (and vice versa), and is more closely related to the
teosinte mtDNAs. Besides the fluidity of the genome structure
due to the high frequency of recombination, the maize
mitochondrial genome presents many other unusual and interesting
features: 1) some genes have a chimeric origin, 2) almost all the
protein gene transcripts are edited, 3)the production of mature
transcripts of some protein genes require transplicing, 4) the
genome has integrated chloroplast DNA sequences which carry some
functional genes, 5) in spite of its large size, the genome does
not contain a full set of tRNA genes. In order to better
understand the unusual organization of the maize mitochondrial
genome as well as its unusual gene organization, we have
initiated large scale sequencing studies.
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