Abstract
From an original A2 allele (colored aleurone), a mutable allele, a2-m-4 1629, that changes from a2 to A2 is described. Mutability is expressed as a very distinct pattern limited to the last cell division.-The mutability of a2-m-4 1629 is autonomously controlled by an En at the a2 locus. This En, inactive on standard a testers for En, is partially active on a2-m-1, an a2 tester for En, and expresses varied levels of activity from limited to nearly full suppression of the a2-m-1 color phenotype.-When the En of the a2-m-4 1629 allele transposes from the a2 locus, it behaves, at the new position, like a standard En in triggering a2-m-1, a-m-1 and a-m(r), which express colored spots on a colorless background. The activity of En is therefore different following the change in chromosome location. This finding supports the "position" hypothesis that has been proposed to explain diverse patterns observed among controlling elements. In this case mutation is related to the terminal cell state and not to tissue differences as shown with some phase-variation regulatory elements.