<P> The lac repressor is a four - part protein, a tetramer, with identical subunits . Each subunit contains a helix - turn - helix (HTH) motif capable of binding to DNA . The operator site where repressor binds is a DNA sequence with inverted repeat symmetry . The two DNA half - sites of the operator together bind to two of the subunits of the repressor . Although the other two subunits of repressor are not doing anything in this model, this property was not understood for many years . </P> <P> Eventually it was discovered that two additional operators are involved in lac regulation . One (O) lies about - 90 bp upstream of O in the end of the lacI gene, and the other (O) is about + 410 bp downstream of O in the early part of lacZ . These two sites were not found in the early work because they have redundant functions and individual mutations do not affect repression very much . Single mutations to either O or O have only 2 to 3-fold effects . However, their importance is demonstrated by the fact that a double mutant defective in both O and O is dramatically de-repressed (by about 70-fold). </P> <P> In the current model, lac repressor is bound simultaneously to both the main operator O and to either O or O . The intervening DNA loops out from the complex . The redundant nature of the two minor operators suggests that it is not a specific looped complex that is important . One idea is that the system works through tethering; if bound repressor releases from O momentarily, binding to a minor operator keeps it in the vicinity, so that it may rebind quickly . This would increase the affinity of repressor for O . </P> <P> The repressor is an allosteric protein, i.e. it can assume either one of two slightly different shapes, which are in equilibrium with each other . In one form the repressor will bind to the operator DNA with high specificity, and in the other form it has lost its specificity . According to the classical model of induction, binding of the inducer, either allolactose or IPTG, to the repressor affects the distribution of repressor between the two shapes . Thus, repressor with inducer bound is stabilized in the non-DNA - binding conformation . However, this simple model cannot be the whole story, because repressor is bound quite stably to DNA, yet it is released rapidly by addition of inducer . Therefore, it seems clear that inducer can also bind to the repressor when the repressor is already bound to DNA . It is still not entirely known what the exact mechanism of binding is . </P>

1. which of the following can be mutated to result in constitutive expression from the lac operon