INVOLVEMENT OF ESCHERICHIA-COLI FIS PROTEIN IN MAINTENANCE OF BACTERIOPHAGE MU-LYSOGENY BY THE REPRESSOR - CONTROL OF EARLY TRANSCRIPTION AND INHIBITION OF TRANSPOSITION

The Escherichia coli FIS (factor for inversion stimulation) protein ha s been implicated in assisting bacteriophage Mu repressor, c, in maint aining the lysogenic state under certain conditions. In a fis strain, a temperature-inducible Mucts62 prophage is induced at lower temperatu res than in a wild-type host (M. Betermier, V. Lefrere, C. Koch, R. Al azard, and M. Chandler, Mol. Microbiol. 3:459-468, 1989). Increasing t he prophage copy number rendered Mucts62 less sensitive to this effect of the fis mutation, which thus seems to depend critically on the lev el of repressor activity. The present study also provides evidence tha t FIS affects the control of Mu gene expression and transposition. As judged by the use of lac transcriptional fusions, repression of early transcription was reduced three- to fourfold in a fis background, and this could be compensated by an increase in cts62 gene copy number. c was also shown to inhibit Mu transposition two- to fourfold less stron gly in a fis host. These modulatory effects, however, could not be cor related to sequence-specific binding of FIS to the Mu genome, in parti cular to the strong site previously identified on the left end. We the refore speculate that a more general function of FIS is responsible fo r the observed modulation of Mu lysogeny.