Effect of Circularity and Superhelicity on Transcription from Bacteriophage λ DNA

We studied RNA synthesis in vitro from closed-circular lambda DNA molecules with varying degrees of superhelicity. The four circular templates examined had 0, -50, -110, and -160 superhelical turns under the conditions of the transcription assay. We found that the total amount of RNA synthesis increases as the template acquires more negative superhelical turns. This increased transcription results from more frequent initiation of RNA chains. Transcription of circular DNA with no superhelical turns appears to mimic RNA synthesis in vivo more closely than transcription from either highly superhelical or linear DNA with regard to two criteria: preferential transcription of the region corresponding to early genes and sensitivity to repression by lambda cI protein. We suggest that the physical basis for the increased initiation of RNA chains from superhelical DNA is the fact that unwinding events are energetically favored on a DNA molecule with negative superhelical turns. Possible general mechanisms are: (a) RNA polymerase must unwind the DNA duplex as a prelude to initiation; (b) the DNA itself must assume a new conformation at the promoter site which requires an unwinding of the DNA duplex.