Analysis of the ends of bacteriophage Mu using site-directed mutagenesis.

We showed previously that two regions at the left end (L1 and L3) and one at the right end (R2) of bacteriophage Mu are essential for transposition. These regions all contain a 22 base-pair sequence with the consensus YGtTTCAYtNNAARYRCGAAAR, where Y and R represent any pyrimidine and purine, respectively. The Mu A protein binds to these regions in vitro, and weakly to sequences between nucleotides 1 and 30 of the right end (R1) and between nucleotides 110 and 135 of the left end (L2). These weak A binding sites contain the sequence AARYRCGAAAR. Here we show, using site-directed mutagenesis, that the weak A binding sites are essential for transposition. Mutations in these weak A binding sites have a greater effect on transposition than mutations of corresponding base-pairs in the stronger A binding sites, located adjacent to these weak A binding sites. We confirm the importance of several of the conserved base-pairs in the consensus sequence YGtTTCAYtNNAARYRCGAAAR. The base-pairs in the A binding sites that are shown to be essential for transposition are all conserved in the ends of the related bacteriophage D108. Furthermore, it is shown that the distance of 90 base-pairs between the two regions at the left end (L1 and L2L3) is essential.