Why Positive Regulators are a Good Idea

Why Positive Regulators are a Good Idea

One of the reasons for positive regulatory systems is even more impor-tant for eukaryotic cells. Let us use the lac regulatory system as the basis for comparison. At any instant, more than half of the lac repressor in a bacterial cell is nonspecifically bound at nonoperator sites. If the cell contained a thousand times as much DNA, then at least a thousand times as much lac repressor would be required to maintain full repres-sion of the lac operon. If the cell contained 20,000 operons regulated by repressors with similar characteristics, then the nucleus could not accommodate the total amount of regulatory proteins required to regu-late these operons. Of course, it may be possible for a repressor to possess greater selectivity than lac repressor has for its operator site, but there has to be an upper limit.

Positive regulatory systems do not require high levels of repressor to maintain low basal expression rates since a positively regulated pro-moter is naturally off. For many genes, the occupancy of only a fraction of the promoter regions by positive regulators may be sufficient to provide sufficient activity to the promoter when it must be turned on.