The protein Mei2 performs at least two functions required in fission yeast for the switch from mitotic to meiotic cell cycles. One of these functions also requires meiRNA. It appears that meiRNA targets Mei2 to the nucleus, where it can promote the first meiotic division. (+info)
Telomerase reverse transcriptase gene is a direct target of c-Myc but is not functionally equivalent in cellular transformation.
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The telomerase reverse transcriptase component (TERT) is not expressed in most primary somatic human cells and tissues, but is upregulated in the majority of immortalized cell lines and tumors. Here, we identify the c-Myc transcription factor as a direct mediator of telomerase activation in primary human fibroblasts through its ability to specifically induce TERT gene expression. Through the use of a hormone inducible form of c-Myc (c-Myc-ER), we demonstrate that Myc-induced activation of the hTERT promoter requires an evolutionarily conserved E-box and that c-Myc-ER-induced accumulation of hTERT mRNA takes place in the absence of de novo protein synthesis. These findings demonstrate that the TERT gene is a direct transcriptional target of c-Myc. Since telomerase activation frequently correlates with immortalization and telomerase functions to stabilize telomers in cycling cells, we tested whether Myc-induced activation of TERT gene expression represents an important mechanism through which c-Myc acts to immortalize cells. Employing the rat embryo fibroblast cooperation assay, we show that TERT is unable to substitute for c-Myc in the transformation of primary rodent fibroblasts, suggesting that the transforming activities of Myc extend beyond its ability to activate TERT gene expression and hence telomerase activity. (+info)
Tight binding of the 5' exon to domain I of a group II self-splicing intron requires completion of the intron active site.
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Group II self-splicing requires the 5' exon to form base pairs with two stretches of intronic sequence (EBS1 and EBS2) which also bind the DNA target during retrotransposition of the intron. We have used dimethyl sulfate modification of bases to obtain footprints of the 5' exon on intron Pl.LSU/2 from the mitochondrion of the alga Pylaiella littoralis, as well as on truncated intron derivatives. Aside from the EBS sites, which are part of the same subdomain (ID) of ribozyme secondary structure, three distant adenines become either less or more sensitive to modification in the presence of the exon. Unexpectedly, one of these adenines in subdomain IC1 is footprinted only in the presence of the distal helix of domain V, which is involved in catalysis. While the loss of that footprint is accompanied by a 100-fold decrease in the affinity for the exon, both protection from modification and efficient binding can be restored by a separate domain V transcript, whose binding results in its own, concise footprint on domains I and III. Possible biological implications of the need for the group II active site to be complete in order to observe high-affinity binding of the 5' exon to domain I are discussed. (+info)
Telomerase activity is sufficient to allow transformed cells to escape from crisis.
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The introduction of simian virus 40 large T antigen (SVLT) into human primary cells enables them to proliferate beyond their normal replicative life span. In most cases, this temporary escape from senescence eventually ends in a second proliferative block known as "crisis," during which the cells cease growing or die. Rare immortalization events in which cells escape crisis are frequently correlated with the presence of telomerase activity. We tested the hypothesis that telomerase activation is the critical step in the immortalization process by studying the effects of telomerase activity in two mortal SVLT-Rasval12-transformed human pancreatic cell lines, TRM-6 and betalox5. The telomerase catalytic subunit, hTRT, was introduced into late-passage cells via retroviral gene transfer. Telomerase activity was successfully induced in infected cells, as demonstrated by a telomerase repeat amplification protocol assay. In each of nine independent infections, telomerase-positive cells formed rapidly dividing cell lines while control cells entered crisis. Telomere lengths initially increased, but telomeres were then maintained at their new lengths for at least 20 population doublings. These results demonstrate that telomerase activity is sufficient to enable transformed cells to escape crisis and that telomere elongation in these cells occurs in a tightly regulated manner. (+info)
The influence of junction conformation on RNA cleavage by the hairpin ribozyme in its natural junction form.
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In the natural form of the hairpin ribozyme the two loop-carrying duplexes that comprise the majority of essential bases for activity form two adjacent helical arms of a four-way RNA junction. In the present work we have manipulated the sequence around the junction in a way known to perturb the global folding properties. We find that replacement of the junction by a different sequence that has the same conformational properties as the natural sequence gives closely similar reaction rate and Arrhenius activation energy for the substrate cleavage reaction. By comparison, rotation of the natural sequence in order to alter the three-dimensional folding of the ribozyme leads to a tenfold reduction in the kinetics of cleavage. Replacement with the U1 four-way junction that is resistant to rotation into the antiparallel structure required to allow interaction between the loops also gives a tenfold reduction in cleavage rate. The results indicate that the conformation of the junction has a major influence on the catalytic activity of the ribozyme. The results are all consistent with a role for the junction in the provision of a framework by which the loops are presented for interaction in order to create the active form of the ribozyme. (+info)
Molecular dynamics studies of U1A-RNA complexes.
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The U1A protein binds to a hairpin RNA and an internal-loop RNA with picomolar affinities. To probe the molecular basis of U1A binding, we performed state-of-the-art nanosecond molecular dynamics simulations on both complexes. The good agreement with experimental structures supports the protocols used in the simulations. We compare the dynamics, hydrogen-bonding occupancies, and interfacial flexibility of both complexes and also describe a rigid-body motion in the U1A-internal loop complex that is not observed in the U1A-hairpin simulation. We relate these observations to experimental mutational studies and highlight their significance in U1A binding affinity and specificity. (+info)
A novel nucleotide incorporation activity implicated in the editing of mitochondrial transfer RNAs in Acanthamoeba castellanii.
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In Acanthamoeba castellanii, most of the mtDNA-encoded tRNAs are edited by a process that replaces one or more of the first three nucleotides at their 5' ends. As a result, base pairing potential is restored at acceptor stem positions (1:72, 2:71, and/or 3:70, in standard tRNA nomenclature) that are mismatched according to the corresponding tRNA gene sequence. Here we describe a novel nucleotide incorporation activity, partially purified from A. castellanii mitochondria, that has properties implicating it in mitochondrial tRNA editing in this organism. This activity is able to replace nucleotides at the first three positions of a tRNA (positions 1, 2, and 3), matching the newly incorporated residues through canonical base pairing to the respective partner nucleotide in the 3' half of the acceptor stem. Labeling experiments with natural (Escherichia coli tRNATyr) and synthetic (run-off transcripts corresponding to A. castellanii mitochondrial tRNALeu1) substrates suggest that the nucleotide incorporation activity consists of at least two components, a 5' exonuclease or endonuclease and a template-directed 3'-to-5' nucleotidyltransferase. The nucleotidyltransferase component displays an ATP requirement and generates 5' pppN... termini in vitro. The development of an accurate and efficient in vitro system opens the way for detailed studies of the biochemical properties of this novel activity and its relationship to mitochondrial tRNA editing in A. castellanii. In addition, the system will allow delineation of the structural features in a tRNA that identify it as a substrate for the labeling activity. (+info)
Photocrosslinking of 4-thio uracil-containing RNAs supports a side-by-side arrangement of domains 5 and 6 of a group II intron.
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Previous studies suggested that domains 5 and 6 (D5 and D6) of group II introns act together in splicing and that the two helical structures probably do not interact by helix stacking. Here, we characterized the major Mg2+ ion- and salt-dependent, long-wave UV light-induced, intramolecular crosslinks formed in 4-thiouridine-containing D56 RNA from intron 5gamma (aI5gamma) of the COXI gene of yeast mtDNA. Four major crosslinks were mapped and found to result from covalent bonds between nucleotides separating D5 from D6 [called J(56)] and residues of D6 near and including the branch nucleotide. These findings are extended by results of similar experiments using 4-thioU containing D56 RNAs from a mutant allele of aI5gamma and from the group IIA intron, aI1. Trans-splicing experiments show that the crosslinked wild-type aI5gamma D56 RNAs are active for both splicing reactions, including some first-step branching. An RNA containing the 3-nt J(56) sequence and D6 of aI5gamma yields one main crosslink that is identical to the most minor of the crosslinks obtained with D56 RNA, but in this case in a cation-independent fashion. We conclude that the interaction between J(56) and D6 is influenced by charge repulsion between the D5 and D6 helix backbones and that high concentrations of cations allow the helices to approach closely under self-splicing conditions. The interaction between J(56) and D6 appears to be a significant factor establishing a side-by-side (i.e., not stacked) orientation of the helices of the two domains. (+info)