The Spm's primary transcript is alternatively spliced to give at least 4 different transcripts which code for proteins with extensive overlapping domains. Two of these proteins, the tnpA and tnpD gene products, are necessary and sufficient to promote dSpm transposition in tobacco. Moreover, the tnpA gene product is also sufficient to suppress the basal level of expression observed in some dSpm-disrupted genes, and probably to activate the expression of inactive elements and of other dSpm-disrupted genes. The functions of the tnpB and tnpC gene products remain unknown. However, their structure suggests that they act as negative regulators of the Spm transposase function.