Open AccessEffect of Circularity and Superhelicity on Transcription from Bacteriophage λ DNA
Author(s) -
Peter Botchan,
James C. Wang,
Harrison Echols
Publication year - 1973
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.70.11.3077
Subject(s) - transcription (linguistics) , dna , rna , transcription bubble , rna polymerase , biology , microbiology and biotechnology , polymerase , duplex (building) , bacteriophage , t7 rna polymerase , nucleic acid , biophysics , gene , genetics , escherichia coli , philosophy , linguistics
We studied RNA synthesisin vitro from closed-circular λ DNA molecules with varying degrees of superhelicity. The four circular templates examined had0, -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 synthesisin 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 λc I 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.