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Example sentences for: pai
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Paramutation has been studied extensively for four maize loci that encode transcriptional regulators of pigment biosynthesis [ 1 ] . Paramutation has also been investigated in the Arabidopsis PAI gene family, at the a1 and chalcone synthase (CHS) transgene loci in petunia, and in a number of other cases [ 2 3 4 ] . The Arabidopsis resistance gene BAL/CPR1 displays a related form of epigenetic instability [ 5 6 ] . Interactions resembling paramutation also occur among non-allelic transgene loci with DNA sequence homology.
In contrast, a trans-methylated singlet PAI2 gene did not trans-methylate a naive allelic PAI2 gene within three generations [ 12 ] . Additional experimentation may eventually provide an answer to the question of what controls the efficiency of such epigenetic interactions.
The methylation status of the maize r1 genes and Arabidopsis PAI genes is altered upon paramutation [ 12 19 ] , while that of maize b1 is not [ 1 ] , except, in the non-conventional way, in the distal paramutation-control region of b1 [ 18 ] . Conversely, extensive exposure of a wild-type Arabidopsis SUPERMAN allele to a heavily methylated and transcriptionally silenced epiallele did not reveal any trans-silencing [ 20 ] .
Likewise, Arabidopsis PAI2 and PAI3 genes that have been trans-methylated by the PAI1/PAI4 locus do not transfer their methylation status to naive singlet genes [ 12 ] . In fact, there are few well-documented cases for the non-allelic relay of trans-silencing ability [ 11 24 ] . Does this amount to an operational difference between allelic (paramutation) and non-allelic (trans-silencing) interactions, or does it simply reflect a lack of data?
In contrast, in the PAI gene family, combining the master locus (WS ecotype PAI1/PAI4 ) with a non-allelic singlet target locus possessing 100% sequence identity (Columbia PAI2 ) resulted in methylation after two generations of heterozygous contact, and methylation became more pronounced after another two generations [ 12 ] . Likewise, in the petunia CHS gene family, interaction between an epiallele of the direct-repeat locus, CHS41, with two types of target loci, either an unlinked inverted repeat locus or a more naïve allele of CHS41, was initially additive and only became suppressive in the second generation [ 4 10 ] .
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