Received July 9, 2002
Accepted August 19, 2002
Inducible DNA Demethylation Mediated by the Maize Suppressor-mutator Transposon-Encoded
TnpA Protein
Hongchang Cui 1 and Nina V. Fedoroff 1*
1
Biology Department, Life Sciences Consortium and Plant Physiology Program, Pennsylvania
State University, University Park, Pennsylvania 16802
* To whom correspondence should be addressed. E-mail: nvf1{at}psu.edu.
Heritable epigenetic inactivation of the maize Suppressor-mutator (Spm
) transposon is associated with promoter methylation, and its reversal is mediated
by the transposon-encoded TnpA protein. We have developed an assay that permits demethylation
of the Spm sequence to be controlled by inducing the expression of TnpA
in plant cells. Using this assay, we show that demethylation is a rapid, active process.
TnpA is a weak transcriptional activator, and deletions that abolish its transcriptional
activity also eliminate its demethylation activity. We show that cell cycle and DNA
synthesis inhibitors interfere with TnpA-mediated Spm demethylation. We
further show that TnpA has a much lower affinity for fully methylated than for hemimethylated
or unmethylated DNA fragments derived from Spm termini. Based on these observations,
we suggest that TnpA binds to the postreplicative, hemimethylated Spm sequence
and promotes demethylation either by creating an appropriate demethylation substrate
or by itself participating in or recruiting a demethylase.
