Transposition of Ac from the P locus of maize into unreplicated chromosomal sites

We have analyzed donor and target sites of the mobile element Activator (Ac) that are altered as a result of somatic transposition from the P locus in maize. Previous genetic analysis has indicated that the two mitotic daughter lineages which result from Ac transposition from P differ in their Ac constitution at the P locus. Both lineages, however, usually contain transposed Ac elements which map to the same genetic position. Using methylation-sensitive restriction enzymes and genomic blot analysis, we identified Ac elements at both the donor P locus and Ac target sites and used this assay to clone the P locus and to identify transposed Ac elements. Daughter lineages were shown to be mitotic descendants from a single transposition event. When both lineages contained Ac genetic activity, they both contained a transposed Ac element on identical genomic fragments independent of the genetic position of the target site. This indicates that in the majority of cases, Ac transposition takes place after replication of the donor locus but before completion of replication at the target site.