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A system where the transposition of MupApl (a derivative of phage Mu carrying a determinant coding for ampicillin resistance) is followed from the small plasmid pML2 into the conjugative plasmid R388 has been used to investigate the influence on Mu transposition of B, an early Mu gene which is involved in normal phage DNA synthesis. In the absence of active B protein a low level (about 1% of normal) of transposition was detected. Roughly a third of these transpositional events was found to lead to the formation of cointegrate DNA structures which were shown to consist of R388, two complete copies of Mu and part only of pML2. The pML2 deletions vary in size but all those investigated appear to originate at an end of Mu. An explanation of these observations is proposed which envisages the B protein as part of the normal transposition complex.

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