Cold temperatures trigger the expression of the CBF family of transcription factors, which in turn activate many downstream genes that confer freezing tolerance to plants. It has been shown previously that the cold regulation of CBF3 involves an upstream bHLH-type transcription factor, ICE1. ICE1 binds to the Myc recognition sequences in the CBF3 promoter. Apart from Myc recognition sequences, CBF promoters also have Myb recognition sequences. We report here that the Arabidopsis MYB15 is involved in cold-regulation of CBF genes and in the development of freezing tolerance. The MYB15 gene transcript is up-regulated by cold stress. The MYB15 protein interacts with ICE1 and binds to Myb recognition sequences in the promoters of CBF genes. Overexpression of MYB15 results in reduced expression of CBF genes whereas its loss-of-function leads to increased expression of CBF genes in the cold. The myb15 mutant plants show increased tolerance to freezing stress whereas its overexpression reduces freezing tolerance. Our results suggest that MYB15 is part of a complex network of transcription factors controlling the expression of CBFs and other genes in response to cold stress.Cold temperatures have a huge impact on the survivability and distribution of living organisms. Plants, being sessile, have evolved efficient mechanisms to sense and adapt to low temperature stress. Plant responses to adverse low temperature are manifested at physiological, molecular and biochemical levels. Many temperate plants have the potential to increase their freezing tolerance after a prior exposure to nonfreezing temperatures, a process known as cold acclimation (1-3). At the molecular level, a specific set of proteins is induced in response to low temperature, which helps plants cope with chilling and freezing stress (4 -8). Proteins induced during cold acclimation include enzymes involved in respiration and metabolism of carbohydrates, lipids, phenylpropanoids, and antioxidants, molecular chaperones, antifreeze proteins, and many others with a presumed function in tolerance to cellular dehydration caused by apoplastic freezing (1, 4, 9).Promoters of many of the cold-responsive genes have the DRE/CRT/LTRE (dehydration responsive element/C-repeat/ low temperature responsive element) sequence, a cis element necessary and sufficient for gene transcription under cold stress (10 -12). The CBF/DREB family of transcription factors binds to this sequence and activates cold-responsive genes (11, 13). The CBF transcription factor genes are also induced by cold, and their induction is regulated by components upstream in the cold response pathways (14 -17). In addition, it has been shown that a loss-of-function mutation in CBF2 results in increased expression of CBF1 and CBF3, implying that CBF2 negatively regulates the expression of CBF1 and CBF3 (18).In addition to the CBF pathway, recent studies have revealed the presence of parallel pathways associated with cold acclimation (19 -21). Some important components mediating cold tolerance through CBF-inde...

An Arabidopsis thaliana mutant, cryophyte, was isolated and found to have an enhanced cold stress-induction of the master regulator of cold tolerance, C-repeat binding factor 2 (CBF2), and its downstream target genes. The mutant is more tolerant to chilling and freezing stresses but is more sensitive to heat stress. Under warm but not cold growth temperatures, the mutant has a reduced stature and flowers earlier. Under long day conditions, flowering of the mutant is insensitive to vernalization. The mutant is also hypersensitive to the phytohormone abscisic acid. The mutation was found in a DEAD box RNA helicase gene that is identical to the previously identified low expression of osmotically responsive genes 4 (LOS4) locus, which was defined by the los4-1 mutation that reduces cold regulation of CBFs and their target genes and renders Arabidopsis plants chilling sensitive. We show evidence suggesting that the CRYOPHYTE/LOS4 protein may be enriched in the nuclear rim. In situ poly(A) hybridization indicates that the export of poly(A) þ RNAs is blocked in the cryophyte/los4-2 mutant at warm or high temperatures but not at low temperatures, whereas the los4-1 mutation weakens mRNA export at both low and warm temperatures. These results demonstrate an important role of the CRYOPHYTE/LOS4 RNA helicase in mRNA export, plant development, and stress responses.

Over one-third of human genome sequence is a product of non-LTR retrotransposition. The retrotransposon that currently drives this process in humans is the highly abundant LINE-1 (L1) element. Despite the ubiquitous nature of L1's in mammals, we still lack a complete mechanistic understanding of the L1 replication cycle and how it is regulated. To generate a genetically amenable model for non-LTR retrotransposition, we have reengineered the Zorro3 retrotransposon, an L1 homolog from Candida albicans, for use in the budding yeast Saccharomyces cerevisiae. We found that S. cerevisiae, which has no endogenous L1 homologs or remnants, can still support Zorro3 retrotransposition. Analysis of Zorro3 mutants and insertion structures suggest that this is authentic L1-like retrotransposition with remarkable resemblance to mammalian L1-mediated events. This suggests that S. cerevisiae has unexpectedly retained the basal host machinery required for L1 retrotransposition. This model will also serve as a powerful system to study the cell biology of L1 elements and for the genetic identification and characterization of cellular factors involved in L1 retrotransposition.