Figure 4.

Expression of repA-luciferase transcriptional fusion in a replication synchronized culture as in Fig. 2. pSM113 is identical to pSP102 except for the luciferase gene downstream of repA. Cultures of PC2/pSM113 were split into two halves. One was maintained at 30°C throughout (□) and the other was subjected to temperature shifts (●). Luciferase activity being heat sensitive is lost irreversibly on incubation at 42°C. It resumes with two peaks on return to 30°C (A) but not in control cultures when the enzyme is made from a vector promoter (B), or when cells were complemented with a dnaC⁺ plasmid to prevent replication synchronization (C), or when repA promoter was repressed with a trans source of RepA (λrepA) (D). Luciferase units are machine units per mg of total protein.