Starvation-induced Mucts62-mediated coding sequence fusion: a role for ClpXP, Lon, RpoS and Crp

The formation of araB-lacZ coding sequence fusions in Escherichia coli is a particular type of chromosomal rearrangement induced by Mucts62, a thermoi nducible mutant of mutator phage Mu. Fusion formation is controlled by the host physiology. It only occurs after aerobic carbon starvation and require s the phage-encoded transposase pA, suggesting that these growth conditions trigger induction of the Mucts62 prophage, Here, we show that thermal indu ction of the prophage accelerated araB-lacZ fusion formation, confirming th at derepression is a rate-limiting step in the fusion process. Nonetheless, starvation conditions remained essential to complete fusions, suggesting a dditional levels of physiological regulation. Using a transcriptional fusio n indicator system in which the Mu early lytic promoter is fused to the rep orter E. coli lacZ gene, we confirmed that the Mucts62 prophage was derepre ssed in stationary phase (S derepression) at low temperature. S derepressio n did not apply to pro-phages that expressed the Mu wild-type repressor, It depended upon the host ClpXP and Lon ATP-dependent proteases and the RpoS stationary phase-specific sigma factor, but not upon Crp, None of these fou r functions was required for thermal induction. Crp was required for fusion formation, but only when the Mucts62 prophage encoded the transposition/re plication activating protein pB, Finally, we found that thermally induced c ultures did not return to the repressed state when shifted back to low temp erature and, hence, remained activated for accelerated fusion formation upo n starvation. The maintenance of the derepressed state required the ClpXP a nd Lon host proteases and the prophage Ner-regulatory protein. These observ ations illustrate how the cts62 mutation in Mu repressor provides the proph age with a new way to respond to growth phase-specific regulatory signals a nd endows the host cell with a new potential for adaptation through the con trolled use of the phage transposition machinery.