Chapter: Genetics and Molecular Biology: Repression and the lac Operon
In vivo Affinity of Repressor for Operator
In vivo
Affinity of Repressor for Operator
The equilibrium dissociation constant for repressor
binding to operator can be determined either from equilibrium measurements or
from the ratio of the association and dissociation rates. The dissociation
constant obtained by either of these methods can also be compared to in vivo data. A correspondence in the
numbers suggests, but does not prove, that the in vitro reactions and conditions closely mimic the in vivo environment.
An estimation of the in vivo equilibrium dissociation constant of repressor for the
operator uses the thousand-fold inducibility of the lac operon. Assume that the operator is completely free of
repressor when the operon is fully induced and that the operator is free of
repressor 1/1000 of the time when the operon is not induced. By definition,
Earlier we discussed the fact that the
concentration of repressor, R, is
approximately 10-8 M. The thousand-fold inducibility means that O/RO is approximately 1/1000. Thus,
This result closely agrees with the values found in vitro in buffers with salt
concentrations equal to the in vivo
value of about 0.15 M.
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