The S/G2‐specific transcription of the human cdc25C gene is due to the periodic occupation of a repressor element (‘cell cycle‐dependent element’; CDE) located in the region of the basal promoter. Protein binding to the major groove of the CDE in G0 and G1 results in a phase‐specific repression of activated transcription. We now show that CDE‐mediated repression is also the major principle underlying the periodic transcription of the human cyclin A and cdc2 genes. A single point mutation within the CDE results in a 10‐ to 20‐fold deregulation in G0 and an almost complete loss of cell cycle regulation of all three genes. In addition, the cdc25C, cyclin A and cdc2 genes share an identical 5 bp region (‘cell cycle genes homology region’; CHR) starting at an identical position, six nucleotides 3′ to the CDE. Strikingly, mutation of the CHR region in each of the three promoters produces the same phenotype as the mutation of the CDE, i.e. a dramatic deregulation in G0. In agreement with these results, in vivo DMS footprinting showed the periodic occupation of the cyclin A CDE in the major groove, and of the CHR in the minor groove. Finally, all three genes bear conspicuous similarities in their upstream activating sequences (UAS). This applies in particular to the presence of NF‐Y and Sp1 binding sites which, in the cdc25C gene, have been shown to be the targets of repression through the CDE.(ABSTRACT TRUNCATED AT 250 WORDS)

SMAD4 (MAD homologue 4 (Drosophila)), also known as DPC4 (deleted in pancreatic cancer), is a tumour suppressor gene that encodes a central mediator of transforming growth factor-beta signalling. Germline mutations in SMAD4 are found in over 50% of patients with familial juvenile polyposis, an autosomal dominant disorder characterized by predisposition to hamartomatous polyps and gastrointestinal cancer. Dense inflammatory cell infiltrates underlay grossly normal appearing, non-polypoid colonic and gastric mucosa of patients with familial juvenile polyposis. This prominent stromal component suggests that loss of SMAD4-dependent signalling in cells within the epithelial microenvironment has an important role in the evolution of intestinal tumorigenesis in this syndrome. Here we show that selective loss of Smad4-dependent signalling in T cells leads to spontaneous epithelial cancers throughout the gastrointestinal tract in mice, whereas epithelial-specific deletion of the Smad4 gene does not. Tumours arising within the colon, rectum, duodenum, stomach and oral cavity are stroma-rich with dense plasma cell infiltrates. Smad4(-/-) T cells produce abundant T(H)2-type cytokines including interleukin (IL)-5, IL-6 and IL-13, known mediators of plasma cell and stromal expansion. The results support the concept that cancer, as an outcome, reflects the loss of the normal communication between the cellular constituents of a given organ, and indicate that Smad4-deficient T cells ultimately send the wrong message to their stromal and epithelial neighbours.

Cold-acclimation-specific (CAS) gene expression has been examined by screening a cDNA library prepared from poly(A)+ RNA of cold-acclimated seedlings of a freezing-tolerant variety of alfalfa (Medcago falcata cv Anik). Three CAS cDNA clones, pSM784, pSM2201, and pSM2358, representing different sequence species, have been used to investigate the relative abundance and time-course of accumulation of corresponding transcripts. Results obtained show that the expression of these CAS genes is regulated in a coordinated manner most likely at the level of transcription. The expression of genes, as measured by mRNA abundance corresponding to the three CAS cDNA clones, is not stimulated or induced by heat shock, water stress, abscisic acid, or wounding. A positive correlation is observed between the expression of these cloned sequences and the degree of freezing-tolerance in four alfalfa cultivars.Freezing temperatures constitute one ofthe most important environmental constraints limiting the productivity and distribution of plants. Although plants are known to differ in their ability to withstand freezing temperatures, the molecular/genetic basis of this differential freezing-tolerance is unclear. It is known, however, that a prior exposure of plants to low nonfreezing temperatures (cold-acclimation) increases their tolerance to subsequent freezing (14). Many physiological and biochemical changes are known to occur in plants during cold-acclimation (5, 10, 13, 21) and it has been suggested (26) Cold-acclimation at 4°C was carried out as described (17) for the time periods mentioned in the text or figure legends. Tests of freezing tolerance were also carried out as described previously (17) except that several temperatures, namely, -50C, -8C, -12C, and -15C were used and LT50 values4 were determined. Administration of StressSeedlings were subjected to water stress, heat shock, wounding, and ABA treatment. ABA was used because it has been implicated in plant responses to environmental stresses (14), particularly to water stress ( 14) and low temperature stress ( 1,3). Water stress was imposed by placing the seedlings in polyethylene glycol-6000 (water potential of -15 bars), heat 4Abbreviations: LT5o, temperature at which 50% seedlings fail to