The tnpA and tnpD gene products of the Spm element are required for transposition in tobacco.

The maize Suppressor-mutator (Spm) element encodes four alternatively spliced transcripts designated tnpA, tnpB, tnpC, and tnpD. tnpA and tnpB are monocistronic, whereas tnpC and tnpD are dicistronic, and the protein-coding sequences of each transcript overlap extensively with those of one or more of the other transcripts. We have analyzed the role of the Spm-encoded gene products in element transposition by using cDNAs with a single open reading frame to (1) complement Spm elements with frameshift mutations and (2) complement each other in a tobacco transposition assay. We report that whereas the tnpA and tnpD gene products are essential for transposition, the tnpB and tnpC gene products are not. We have analyzed the structure of empty donor sites, new insertion sites, and potential transposition intermediates. We discuss the implications of our findings for the mechanism of Spm transposition.