Genetic analysis of the functions of the transposable element En in Zea mays: limited transposase elicits a differential response on reporter alleles.

The En/Spm transposable element has 13 exons. Eleven of the exons contribute to the 2.5-kb transcript (tnpA) that encodes the TNPA protein. The other two large exons contribute to a 6-kb transcript (tnpD) that encodes the TNPD protein. The TNPA protein conditions the genetically determined suppressor function (S) and the TNPD protein along with the TNPA protein provides the mutator (M) function. The limits of the DNA En/Spm element sequences responsible for the two functions (S and M) have previously been tested by studies with transgenic systems and two mutant derivatives of En/Spm. Experiments reported here expand on the conclusions derived from studies with the two mutant derivatives En2 and Spm-w 8011. By using an appropriate reporter allele, the mutator function of En2, though impaired, shows a perceptible mutator expression. A less impaired (partially deleted vs. complete subterminal motifs) reporter allele will elicit expression from a limited amount of transposase. This demonstrates that the carboxy terminus is not essential for M function. The suppressor function of En2 is limited when various doses of both transposase-contributing alleles, as well as reporter alleles, are tested. The basis of the differences between suppressible and nonsuppressible alleles is also discussed.