We have isolated and sequenced a full-length cDNA clone encoding rat glyceraldehyde-3-phosphate-dehydrogenase (GAPDH, E.C.1.2.1.12). The entire mRNA is 1269 nucleotides long exclusive of poly(A) and contains respectively 71 and 196 bases of 5' and 3' non-coding regions. Primer extension as well as S1 nuclease protection experiments clearly established that a single (or at least a highly prominent) GAPDH mRNA species is expressed in all rat tissues examined. This sequence allowed the determination of the hitherto unknown primary structure of rat GAPDH which is 333 aminoacids long. Comparison between GAPDH sequences from rat, man and chicken revealed a high degree of sequence conservation at both nucleotide and protein levels.

We have isolated and identified cDNA clones containing part of the coding sequence for rat glyceraldehyde-3-phosphate-dehydrogenase (GAPDH, E.C. 1.2.1.12). By using one of these clones as a probe, we have shown that: i) the abundance of GAPDH mRNA is different in various tissues of the adult rat and in good correlation with the abundance of the enzyme; ii) the transcription rates are quite similar in all tissues tested. We therefore conclude that the tissue-specific differential GAPDH gene expression is regulated by adjusting the abundance of its mRNA at the post-transcriptional level.

Nucleolar localization of vertebrate box C/D snoRNA involves transit through Cajal bodies, but the signi®-cance of this event is unkown. To de®ne better the function of this compartment, we analyzed here the maturation pathway of mammalian U3. We show that 3¢-extended U3 precursors possess a mono-methylated cap, and are not associated with ®brillarin and hNop58. Importantly, these precursors are detected at both their transcription sites and in Cajal bodies. In addition, mature U3, the core box C/D proteins and the human homolog of the methyltransferase responsible for U3 cap tri-methylation, hTgs1, are all present in Cajal bodies. In yeast, U3 follows a similar maturation pathway, and equivalent 3¢-extended precursors are enriched in the nucleolus and in the nucleolar body, a nucleolar domain that concentrates Tgs1p under certain growth conditions. Thus, spatial organization of U3 maturation appears to be conserved across evolution, and involves specialized and related nuclear compartments, the nucleolus/nucleolar body in yeast and Cajal bodies in higher eukaryotes. These are likely places for snoRNP assembly, 3¢ end maturation and cap modi®cation.

Transcription of the gene coding for cyclin A, a protein required for S-phase transit, is cell cycle regulated and is restricted to proliferating cells. To further explore transcriptional regulation linked to cell division cycle control, a genomic clone containing 5 flanking sequences of the murine cyclin A gene was isolated. When it was fused to a luciferase reporter gene, it was shown to function as a proliferation-regulated promoter in NIH 3T3 cells. Transcription of the mouse cyclin A gene is negatively regulated by arrest of cell proliferation. A mutation of a GC-rich sequence conserved between mice and humans is sufficient to relieve transcriptional repression, resulting in a promoter with constitutively high activity. In agreement with this result, in vivo footprinting reveals a protection of the cell cycle-responsive element in G 0 /early G 1 cells which is not observed at later stages of the cell cycle. Moreover, the footprint is present in dimethyl sulfoxide-induced differentiating and not in proliferating Friend erythroleukemia cells. Conversely, two other sites, which in vitro bind ATF-1 and NF-Y, respectively, are constitutively occupied throughout cell cycle progression.Cyclins are a family of proteins involved in the control of the eukaryotic cell division cycle. As regulatory subunits of a particular subset of periodically activated protein kinases, they control the temporal transition between the various stages of the cell cycle (for reviews, see references 18, 38, 48, 49, 59, and 60). Their dysregulation has also been proposed to be associated with the genesis of some human cancers (8,23,26,31,32,52,56,67,68). The A-type cyclin has been implicated in the control of the entry into S phase, as well as in the G 2 /M transition (22,47,53,66), by binding to the cdk2 and cdc2 kinases, respectively (51, 65). Cyclin A mutants of Drosophila embryos arrest in G 2 (36, 37). In addition to its role in the dependence of mitosis on S-phase completion, it may have a role in mitotic control (37,63,66). Cyclin A is found in cellular activities which promote both replication (10,19,47,50,53,62) and transcription (for reviews, see references 35,45,69). In the former case, it has been shown to be associated with replication complexes and is likely to directly participate in the phosphorylation of replication protein A (RPA) (20,70). In the latter case, its association with the E2F/DRTF1 family of transcription factors has been proposed to play a central role in cell cycle transcriptional control. A large body of evidence suggests that cyclin-dependent kinases participate in the regulation of the activity of this transcription complex. This is believed to occur either directly, by the phosphorylation of E2F by cyclin A-cdk2 kinase, or indirectly through the phosphorylation of a group of proteins structurally related to RB, the retinoblastoma susceptibility gene product, by cyclins D, E, and A, which are associated with cdk4, cdk6, and cdk2 kinases. In agreement with these ideas, cyclin A is found complexed with E2...