Temporal control of transposition in Tn5.

IS50R is an insertion sequence associated with the transposon Tn5. IS50R carries the structural genes for two proteins; one (P1) is the Tn5 transposase, and the other (P2) is an inhibitor of transposition. These two proteins are translated from two different transcripts, m1 and m2. When bacteriophage lambda::IS50R DNA was introduced into a bacterial cell, m1 and m2 were initially at relative levels of about 1 to 2. As time progressed the amount of m1 fell, whereas the amount of m2 continued to increase, until after about 3 h the ratio of m1 to m2 was about 1 to 80. The temporal changes in the levels of these transcripts correlated with temporal changes in P1 and P2 levels and Tn5 transposition that have been documented in other studies. We measured the stability of the messages and showed that the differences in the levels of m1 and m2 must reflect real differences in the strengths of their promoters and that the changes in transcription kinetics are mediated by the dam methylation system of the cell and are not determined by IS50R products. Our results show that the 5' end of m2 is about twice as stable as that of m1, which raises the possibility that differential message stability does, in part, influence the ratio of inhibitor to transposase.