Fig. 3.

Effect of the circadian clock on day and night expression patterns of clock, excision repair, and checkpoint proteins. (A–C) Extracts from brains harvested at ZT18 (night) and ZT06 (day) were analyzed by immunoblotting. (A) Clock proteins. Clock shows no oscillation, whereas Cry1 and Per2 oscillate antiphase with Bmal1. (B) Nucleotide excision repair proteins. XPA is highly expressed during the day and down-regulated at night. The other excision repair proteins do not exhibit a measurable variation between ZT18 and ZT06. (C) Checkpoint proteins. None of the checkpoint/cell cycle proteins tested exhibits a measurable difference between day (ZT06) and night (ZT18) values. (D) Circadian oscillation of XPA. Brains were harvested from mice under either LD or DD conditions, and the levels of XPA over a circadian period were analyzed by immunoblotting along with the four core clock proteins as a reference. Note the zenith values for XPA at afternoon/evening hours under both ZT and CT conditions are in phase with Bmal1 and antiphase with Cry1 and Per2 as expected for a Clock·Bmal1-controlled protein. Clock that does not oscillate is used as a loading control for a nonoscillating protein. (E) Quantitative analysis of XPA oscillation. Each data point represents XPA value from the cerebrum of one mouse, and the values are expressed relative to that of XPA at CT14, which was the highest.