THE GENERATION OF MUTATOR TRANSPOSABLE ELEMENT SUBFAMILIES IN MAIZE

The mobile DNAs of the Mutator system of maize (Zea mays) are exceptio nal both in structure and diversity. So far, six subfamilies of Mu ele ments have been discovered; all Mu elements share highly conserved ter minal inverted repeats (TIRs), but each sub-family is defined by inter nal sequences that are apparently unrelated to the internal sequences of any other Mu subfamily. The Mu1/Mu2 subfamily of elements was creat ed by the acquisition of a portion of a standard maize gene (termed MR S-A) within two Mu TIRs. Beside the unusually long (185-359bp) and div erse TIRs found on all of these elements, other direct and inverted re peats are often found either within the central portion of a Mu elemen t or within a TIR. Our computer analyses have shown that sequence dupl ications (mostly short direct repeats interrupted by a few base pairs) are common in non-autonomous members of the Mutator, Ac/Ds, and Spm(E n) systems. These duplications are often tightly associated with the e lement-internal end of the TIRs. Comparisons of Mu element sequences h ave indicated that they share more terminal components than previously reported; all subfamilies have at least the most terminal 215 bp, at one end or the other, of the 359-bp Mu5 TIR. These data suggest that m any Mu element subfamilies were generated from a parental element that had termini like those of Mu5. With the Mu5 TIRs as a standard, it wa s possible to determine that elements like Mu4 could have had their un usual TIRs created through a three-step process involving (1) addition of sequences to interrupt one TIR, (2) formation of a stem-loop struc ture by one strand of the element, and (3) a subsequent DNA repair/gen e conversion event that duplicated the insertion(s) within the other T IR. A similar repair/conversion extending from a TIR stem into loop DN A could explain the additional inverted repeat sequences added to the internal ends of the Mu4 and Mu7 TIRs. This same basic mechanism was f ound to be capable of generating new Mu element subfamilies. After end onucleolytic attack of the loop within the stem-loop structure, repair /conversion of the gap could occur as an intermolecular event to gener ate novel internal sequences and, therefore, a new Mu element subfamil y. Evidence supporting and expanding this model of new Mu element subf amily creation was identified in the sequence of MRS-A.