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(cytosine DNA
methyltransferase|mutagenesis|RIP|silencing|
Institute of Molecular Biology, University of Oregon, Eugene, OR
97403-1229
Edited by David D. Perkins, Stanford University, Stanford, CA,
and approved May 6, 2002 (received for review April 9, 2002)
During sexual development, Neurospora crassa
inactivates genes in duplicated DNA segments by a hypermutation
process, repeat-induced point mutation (RIP). RIP introduces C:G to T:A
transition mutations and creates targets for subsequent DNA methylation
in vegetative tissue. The mechanism of RIP and its relationship to DNA
methylation are not fully understood. Mutations in DIM-2, a DNA
methyltransferase (DMT) responsible for all known cytosine methylation
in Neurospora, does not prevent RIP. We used RIP
to disrupt a second putative DMT gene in the Neurospora
genome and tested mutants for defects in DNA methylation and RIP. No
effect on DNA methylation was detected in the tissues that could be
assayed, but the mutants showed recessive defects in RIP. Duplications
of the am and mtr genes were completely stable in crosses homozygous for the mutated potential DMT gene, which
we call rid (RIP
defective). The same duplications were inactivated
normally in heterozygous crosses. Disruption of the rid
gene did not noticeably affect fertility, growth, or development. In
contrast, crosses homozygous for a mutation in a related gene in
Ascobolus immersus, masc1, reportedly
fail to develop and heterozygous crosses reduce methylation induced
premeiotically [Malagnac, F., Wendel, B., Goyon, C., Faugeron, G.,
Zickler, D., et al. (1997) Cell 91, 281-290]. We isolated homologues of rid from
Neurospora tetrasperma and Neurospora
intermedia to identify conserved regions. Homologues possess
all motifs characteristic of eukaryotic DMTs and have large distinctive
C- and N-terminal domains.
Genetics
A cytosine methyltransferase homologue is essential for
repeat-induced point mutation in Neurospora crassa
epigenetics)
*
To whom reprint requests should be addressed. E-mail:
selker{at}molbio.uoregon.edu.
www.pnas.org/cgi/doi/10.1073/pnas.132212899
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