On the interaction of actinomycin and DNA
<p>Experiments have been accomplished that (a) further define the nature of the strong, G-containing DNA binding sites for actinomycin D (AMD), and (b) quantitate the in vitro inhibition of E. coli RNA polymerase activity by T7 DNA-bound AMD.</p> <p>Twenty-five to forty percen...
| Summary: | <p>Experiments have been accomplished that (a) further define the nature of the strong, G-containing DNA binding sites for actinomycin D (AMD), and (b) quantitate the in vitro inhibition of E. coli RNA polymerase
activity by T7 DNA-bound AMD.</p>
<p>Twenty-five to forty percent of the G's of crab dAT are disallowed
as strong AMD binding sites. The G's are measured to be
randomly distributed, and, therefore, this datum cannot be explained
on the basis of steric interference alone. Poly dAC:TG binds as much
AMD and as strongly as any natural DNA, so the hypothesis that the
unique strong AMD binding sites are G and a neighboring purine is
incorrect. The datum can be explained on the basis of both steric
interference and the fact that TGA is a disallowed sequence for strong
AMD binding.</p>
<p>Using carefully defined in vitro conditions, there is one RNA
synthesized per T7 DNA by E. coli RNA polymerase. The rate of the
RNA polymerase-catalyzed reaction conforms to the equation
1/rate = 1/k<sub>A</sub>(ATP) + 1/K<sub>G</sub>(GTP) + 1/K<sub>C</sub>(CTP) + 1/K<sub>U</sub>(UTP)
T7 DNA-bound AMD has only modest effects on initiation and termination
of the polymerase-catalyzed reaction, but a large inhibitory effect
on propagation. In the presence of bound AMD, k<sub>G</sub> and k<sub>C</sub> are decreased,
whereas k<sub>A</sub> and k<sub>U</sub> are unaffected. These facts are interpreted to mean that on the microscopic level, on the average, the rates of incorporation of ATP and UTP are the same in the absence or
presence of bound AMD, but that the rates of incorporation of GTP and
CTP are decreased in the presence of AMD.</p>
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