FIGURE 4.

Compared with the other alleles, DBT^ar and DBT^K38R extend the half-life of PER. Normalized PER-YFP levels in individual cells (top panel, thin gray line) co-expressing (A) DBT^ar (n = 56 cells), (B) DBT^K38R (n = 62 cells), or (C) no exogenous DBT (n = 53 cells). In each instance, the average PER intensity versus time and its standard error are shown in thick lines, top and middle panels, respectively. The measurements show PER has a half-life of about 5.4 h and 6.5 h in the presence of DBT^ar and DBT^K38R, respectively, and 5.1 h with endogenous DBT. PER half-lives with DBT^ar and DBT^K38R are both severalfold longer than with DBT, DBT^S, or DBT^L. Unlike in Figs. 2C and 3C, the fraction of left-over PER after interaction with these DBT alleles is found to depend strongly on the initial accumulation of the protein in the cell (bottom panels). In all three cases, cells expressing large amounts of PER show stable levels of the protein even ∼10 h after induction.