Problems associated with dose response in steroid-hormone activation of structural genes.

A typical target cell for a sex steroid hormone contains 10 000--20 000 specific high-affinity receptors for that hormone. However full physiological responses can be achieved with only 2000 of these receptors involved in hormone--receptor complex interaction with the nucleus. The number of nuclear acceptor sites that must be filled before responses occur maybe even less. This implies that multiple occupation of nuclear acceptor sites by hormone--receptor may occur permitting co-operative induction of transcription of selected genes. The numbers of sites of initiation of RNA synthesis seem excessively high (about 70 000 per cell). Although this may be an artifact of the isolation procedures the proportion of initiation sites under hormonal control (equivalent to about 30 000 per cell) is still large. The numbers of mRNA species under hormonal control varies greatly depending on the particular hormone and target tissue. The extent to which these different observations can be incorporated into a unifying theory is discussed.