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Microbiol. Rev., 12 1995, 673-685, Vol 59, No. 4
AJ Griffiths
Among eukaryotes, plasmids have been found in fungi and plants but not in
animals. Most plasmids are mitochondrial. In filamentous fungi, plasmids
are commonly encountered in isolates from natural populations. Individual
populations may show a predominance of one type, but some plasmids have a
global distribution, often crossing species boundaries. Surveys have shown
that strains can contain more than one type of plasmid and that different
types appear to be distributed independently. In crosses, plasmids are
generally inherited maternally. Horizontal transmission is by cell contact.
Circular plasmids are common only in Neurospora spp., but linear plasmids
have been found in many fungi. Circular plasmids have one open reading
frame (ORF) coding for a DNA polymerase or a reverse transcriptase. Linear
plasmids generally have two ORFs, coding for presumptive DNA and RNA
polymerases with amino acid motifs showing homology to viral polymerases.
Plasmids often attain a high copy number, in excess of that of
mitochondrial DNA. Linear plasmids have a protein attached to their 5' end,
and this is presumed to act as a replication primer. Most plasmids are
neutral passengers, but several linear plasmids integrate into
mitochondrial DNA, causing death of the host culture. Inferred amino acid
sequences of linear plasmid ORFs have been used to plot phylogenetic trees,
which show a fair concordance with conventional trees. The circular
Neurospora plasmids have replication systems that seem to be evolutionary
intermediates between the RNA and the DNA worlds.
Copyright © 1995, American Society for Microbiology
Natural plasmids of filamentous fungi
Botany Department, University of British Columbia, Vancouver, Canada.
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