Somatic inactivation and reactivation of Ac associated with changes in cytosine methylation and transposase expression.

Several metastable Ac alleles at the maize p locus were identified that produced novel pericarp variegation patterns. From the transmission analysis of pericarp sectors, we show that Ac inactivation is a somatic process. Ac excision from P and transactivation of an unlinked Ds was delayed or absent in plants with metastable Ac alleles. These decreases in Ac activity were correlated to increases in cytosine methylation at specific sites near the start of Ac transcription (open reading frame a; ORFa) and at sites in some flanking P sequence. Reactivation of inactive alleles was accompanied by decreased methylation of Ac and P sequences. Using a competitive polymerase chain reaction assay, steady state levels of ORFa transcript were quantitatively compared among the various metastable alleles. We propose that changes in the methylation profile of Ac correspond to changes in Ac activity through the differential accumulation of Ac transcript.