Two Classes of Tn10 Transposase Mutants That Suppress Mutations in the Tn10 Terminal Inverted Repeat

Tn10 transposition requires IS10 transposase and essential sequences at the two ends of the element. Mutations in terminal basepairs 6-13 confer particularly strong transposition defects. We describe here the identification of transposase mutations that suppress the transposition defects of such terminus mutations. These mutations are named ``SEM'''' for suppression of ends mutations. All of the SEM mutations suppress more than a single terminus mutation and thus are not simple alterations of transposase/end recognition specificity. The mutations identified fall into two classes on the basis of genetic tests, location within the protein and nature of the amino acid substitution. Class I mutations, which are somewhat allele specific, appear to define a small structural and functional domain of transposase in which hydrophobic interactions are important at an intermediate stage of the transposition reaction, after an effective interaction between the ends but before transposon excision. Class II mutations, which are more general in their effects, occur at a single residue in a small noncritical amino-terminal proteolytic domain of transposase and exert their affects by altering a charge interaction; these mutations may affect act early in the reaction, before or during establishment of an effective interaction between the ends.