C1 REPRESSOR-MEDIATED DNA LOOPING IS INVOLVED IN C1 AUTOREGULATION OFBACTERIOPHAGE-P1

C1 repressor is required to repress the lytic functions of a P1 propha ge in vivo, Transcription of the cl gene is autoregulated via the C1-c ontrolled operator Op99a,b which overlaps the promoter of the el gene. It is negatively affected by Lxc: corepressor and the DNA region upst ream of c1, which contains the additional operators Op99c, d, and e. W e have explored these effects by constructing a set of lacZ reporter p lasmids with Op99a,b and varying parts of the upstream DNA region, Tra nscription levels were measured in vivo with a two-plasmid system cont aining the lacZ reporter and a c1(+) lxc(+) or c1(+) lxc - plasmid, Co mpared to the C1+Lxc-repressed lacZ reporter with all operators presen t, the basal level of beta-galactosidase activity increases successive ly when (i) upstream operators were deleted or inactivated, (ii) Lxc c orepressor was removed, and (iii) C1 and Lxc were absent. By that mean s a 2 x 2 x 15-fold stepwise increase in enzyme activity was found, Us ing electron microscopy to visualize the interaction of C1 repressor w ith the operators in vitro, looped DNA molecules were observed. Althou gh all operators can participate in C1-mediated DNA looping, loops bet ween Op99a,b and Op99d occurred predominantly. Lxc is not required but increases drastically the frequency of loop formation. The results in dicate that C1-mediated DNA looping may be a second element besides Lx c for fine-tuning the autoregulation of c1 transcription,