Genetic determination of kinetic parameters in beta-glucuronidase induction by androgen

Abstract

A regulatory locus, Gus-r, determines the rate and extent of androgen inducibility of beta-glucuronidase in mouse kidney epithelial cells. The kinetics of induction are strikingly similar when enzyme concentration, rates of enzyme synthesis, and beta-glucuronidase mRNA are measured. After an initial lag period the accumulation of mRNA activity obeys simple turnover kinetics defined by ka, a zero order rate constant for acquisition of mRNA activity, and kb, a first order rate constant for loss of activity. The induced state is approached with a half-life of 8-9 days in the presence of testosterone and decays rapidly in the absence of testosterone. The half-life of both beta-glucuronidase and its mRNA appear to be much shorter, approximately 1-2 days, in both the presence and absence of testosterone. We conclude that the material accumulating in response to androgen is probably a transcriptionally activated state of beta-glucuronidase chromatin. Comparison of the a and b alleles of Gus-r, and a newly described h allele, shows that Gus-r determines ka and the duration of the lag period, but not kb, which was genetically invariant. The changes in ka and the duration of the lag are inversely related, suggesting that they reflect a common step during induction. These results are most simply accounted for by assuming that beta-glucuronidase regions in chromatin react with many molecules of androgen receptor protein-testosterone complex and that the rate of transcription is a function of the number of molecules bound. The lag period, then, reflects a requirement that a minimum number of sites must be occupied before transcription begins to increase. We suggest that the Gus-r locus determines the accessibility or affinity of androgen receptor complexes to chromatin. Because of this Gus-r determine both ka and the duration of the lag and the two parameters are inversely related to each other.