FIG 7.
A suboptimal concentration of F55 allows the derepression of the target genes. Schematic representations of the infected cell and of the UV-inducible region of SSV1 genome are presented. The operators recognized by F55 are in green, yellow, and blue. Bent arrows indicate the transcription start sites, and dashed lines represent transcripts. Dimers of F55 are represented by purple ovals. In the lysogenic cell, the amount of F55 is suitable to saturate most of its binding sites and to keep SSV1 in a steady carrier state. At 2 h postirradiation, a decrease of about 50% of the F55 concentration and a concurrent increase of the viral copy number led to the dissociation from the lower-affinity operators in the promoter of Tind and Tlys. Later, at 4 h postirradiation, the dilution effect is enhanced by a further accumulation of the viral DNA, which results in the release of the early promoters (i.e., those of T5 and T6), thus allowing transcription derepression.