Genetic and molecular analysis of a three-component transposable-element system in maize.

Two different factors control the mutability of an unstable allele (c2-m881058Y) of the C2 gene of maize. Both an autonomous En/Spm element and an unrelated independent factor, named Mediator, are coordinately required for the excision of the insert in c2-m881058Y. According to genetic analysis, Mediator does not have the suppressor (S) function or mutator (M) function of En/Spm. Mediator has no effect on the timing or frequency of excision of En1, En-low, or various I/dSpm elements. Hence, Mediator only mediates a specific interaction between En and the insert at c2-m881058Y. Molecular analysis of c2-m881058Y has revealed a 3.3 kb, complex, En-related receptor element inserted into the second exon of the C2 gene. The ends of this element are homologous to the ends of En/Spm, but an internal 1.7 kb region shows no En/Spm homology. A great degree (11-14%) of nucleotide changes, relative to En1, occur within and between the 12 bp TNPA binding motifs. Alterations of these critical cis-determinants may account for the need for a "helper" factor for excision. This element is named Irma, for Inhibitor that requires Mediator also, and represents a unique, low copy number class of receptor element.