Bmal1
Organisms from flies to humans have daily circadian
rhythms entrained with the 24-hour cycle of day and night that regulate many
physiological systems. In mammals, there appears to be a master regulator of
circadian rhythms in the hypothalamus, as well as additional peripheral
mechanisms. The basic framework of the molecular pathways in the cell that
produce and maintain these daily rhythms has been elucidated. The central
feature of the circadian pathways are two transcription factors, Bmal1 and
Clock, that are expressed in a 24 hour cycle and that act together as a
heterodimer to regulate the expression of other genes involved in maintaining
the circadian rhythm. Bmal1 and Clock activate transcription of multiple Period
(per) and Cryptochrome (Cry) genes. When the activity of Bmal1 and Clock is
maximal, Per and Cry genes are transcribed, and then translated in the
cytoplasm. The translated Per and Cry genes form a complex in the cytoplasm
that is transported back into the nucleus. In the nucleus, Per and Cry form a
feedback loop that inhibits transcriptional activation by Bmal1 and Clock,
lowering their own expression in the process. At the same time, Per and Cry
increase transcription of Bmal1 and Clock genes, to initiate the next phase of
the cycle once again. This cycle runs autonomously in the cell, causing jet lag
during travel, but can be regulated to retrain to changes in the light cycle.
Factors that regulate the cycle include casein kinase 1 epsilon (CK1e) that
phosphorylates Per2 and induces its degradation. Mutation of PER2 in humans has
been genetically linked to changes in human sleep patterns. Further exploration
of the factors that regulate circadian rhythms may enable manipulation of the
system in sleep disorders.
We have investigated the genetic interactions between cry2 and the various flowering pathways in relation to the regulation of flowering by photoperiod and vernalization. For this, we combined three alleles of CRY2, the wild-type CRY2-Landsberg erecta (Ler), a cry2 loss-of-function null allele, and the gain-of-function CRY2-Cape Verde Islands (Cvi), with mutants representing the various photoreceptors and flowering pathways. The analysis of CRY2 alleles combined with photoreceptor mutants showed that CRY2-Cvi could compensate the loss of phyA and cry1, also indicating that cry2 does not require functional phyA or cry1. The analysis of mutants of the photoperiod pathway showed epistasis of co and gi to the CRY2 alleles, indicating that cry2 needs the product of CO and GI genes to promote flowering. All double mutants of this pathway showed a photoperiod response very much reduced compared with Ler. In contrast, mutations in the autonomous pathway genes were additive to the CRY2 alleles, partially overcoming the effects of CRY2-Cvi and restoring day length responsiveness. The three CRY2 alleles were day length sensitive when combined with FRI-Sf2 and/or FLC-Sf2 genes, which could be reverted when the delay of flowering caused by FRI-Sf2 and FLC-Sf2 alleles was removed by vernalization. In addition, we looked at the expression of FLC and CRY2 genes and showed that CRY2 is negatively regulated by FLC. These results indicate an interaction between the photoperiod and the FLC-dependent pathways upstream to the common downstream targets of both pathways, SOC1 and FT.