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Research Article 8: Is Subunit Assembly a Rate-Limiting Factor on Induction?

Lucille Adamson, Carol Gross, Aaron Novick


The basal or uninduced rate of formation of β-galactosidase is extremely low and can be accounted for by the production of less than one messenger for β-galactosidase per cell per generation (Rotman, pers. commun.). Since the concentration of subunits resulting from one messenger molecule per cell would be less than 10−7M, we wondered whether the assembly of these subunits to the active tetrameric enzyme would require a relatively long time. Were a long time necessary, one might expect to see an increase in the time required (the lag) to establish the ultimate rate of β-galactosidase synthesis when a culture is induced with low concentrations of inducer.

The lag on induction should include in addition to the time for assembly of β-galactosidase subunits contributions from (a) the time needed after addition of inducer for the fraction of operators free of repressor to reach steady state values, and (b) the time to transcribe and translate the gene for β-galactosidase. An effect of slow assembly would be seen only if the other two processes were relatively fast and independent of the rate of synthesis of β-galactosidase.

Unhappily, there is lack of agreement in published reports on the observed values of the lag at low rates of synthesis. In one case, it has been reported that the lag did not increase as the inducer concentration was reduced below saturation values (Pardee and Prestidge, 1961). In another case, it has been reported that the lag did increase at lower concentrations of inducer (Boezie and...

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