Structure of the Tetrahymena thermophila telomerase RNA helix II template boundary element

Telomere addition by telomerase requires an internal templating sequence located in the RNA subunit of telomerase. The correct boundary definition of this template sequence is essential for the proper addition of the nucleotide repeats. Incorporation of incorrect telomeric repeats onto the ends of chromosomes has been shown to induce chromosomal instability in ciliate, yeast and human cells. A 5′ template boundary defining element (TBE) has been identified in human, yeast and ciliate telomerase RNAs. Here, we report the solution structure of the TBE element (helix II) from Tetrahymena thermophila telomerase RNA. Our results indicate that helix II and its capping pentaloop form a well-defined structure including unpaired, stacked adenine nucleotides in the stem and an unusual syn adenine nucleotide in the loop. A comparison of the T.thermophila helix II pentaloop with a pentaloop of the same sequence found in the 23S rRNA of the Haloarcula marismortui ribosome suggests possible RNA and/or protein interactions for the helix II loop within the Tetrahymena telomerase holoenzyme.     

Reprogramming of telomerase by expression of mutant telomerase RNA template in human cells leads to altered telomeres that correlate with reduced cell viability.

Telomerase synthesizes telomeric DNA by copying the template sequence of its own RNA component. In Tetrahymena thermophila and yeast (G. Yu, J. D. Bradley, L. D. Attardi, and E. H. Blackburn, Nature 344:126-131, 1990; M. McEachern and E. H. Blackburn, Nature 376:403-409, 1995), mutations in the template domain of this RNA result in synthesis of mutant telomeres and in impaired cell growth and survival. We have investigated whether mutant telomerase affects the proliferative potential and viability of immortal human cells. Plasmids encoding mutant or wild-type template RNAs (hTRs) of human telomerase and the neomycin resistance gene were transfected into human cells to generate stable transformants. Expression of mutant hTR resulted in the appearance of mutant telomerase activity and in the synthesis of mutant telomeres. Transformed cells were not visibly affected in their growth and viability when grown as mass populations. However, a reduction in plating efficiency and growth rate and an increase in the number of senescent cells were detected in populations with mutant telomeres by colony-forming assays. These results suggest that the presence of mutant telomerase, even if coexpressed with the wild-type enzyme, can be deleterious to cells, likely as a result of the impaired function of hybrid telomeres.     

Expressing Hoxa2 across the entire endochondral skeleton alters the shape of the skeletal template in a spatially restricted fashion

Hox genes control morphogenesis along the antero-posterior axis. The skeleton of vertebrates offers an exemplar readout of their activity: Hox genes control the morphology of the skeleton by defining type of vertebrae, and structure of the limbs. The head skeleton of vertebrates is formed by cranial neural crest (CNC), and mainly by a Hox-free domain of the CNC. Ectopic expression of anterior Hox genes in the CNC prevents the formation of the facial skeleton. These inhibitory effects on skeletogenesis are at odds with the recognized function of Hox genes in patterning the developing skeleton. To clarify these controversial effects, we overexpressed Hoxa2 across the entire developing endochondral skeleton in mouse. This gave rise to strong and spatially restricted effects: the most noticeable abnormalities were detected in the cranial base and consisted in a failure of bones to form or in a transformed morphology of bones. The rest of the skeleton exhibited milder defects, which never consisted in the absence or the transformation of any skeletal components. Analyses at early stages of endochondral bone development showed disorganized cell condensations in the cranial base of Col2a1-Hoxa2 transgenic embryos. We show that the distribution of Hoxa2-positive cells in Col2a1-Hoxa2 embryos does not match the wild-type developing cartilages. The Hoxa2-positive cells detected in atypical, non-chondrogenic location in the cranial base, remain as chondrocytes and lay down cartilage, indicating that Hoxa2 does not alter the fate of chondrocytes, but interferes with their spatial distribution. We propose that the ability of Hoxa2 to change the spatial distribution of cells accounts for the different phenotypes observed in Col2a1-Hoxa2 embryos; it also provides an explanation for the apparent inconsistency between the inhibitory effects of Hoxa2 on skeletal development, and the ability of Hox genes to establish the morphology of the vertebrate skeleton.     

Bacterial protein translocation requires only one copy of the SecY complex in vivo

The transport of proteins across the plasma membrane in bacteria requires a channel formed from the SecY complex, which cooperates with either a translating ribosome in cotranslational translocation or the SecA ATPase in post-translational translocation. Whether translocation requires oligomers of the SecY complex is an important but controversial issue: it determines channel size, how the permeation of small molecules is prevented, and how the channel interacts with the ribosome and SecA. Here, we probe in vivo the oligomeric state of SecY by cross-linking, using defined co- and post-translational translocation intermediates in intact Escherichia coli cells. We show that nontranslocating SecY associated transiently through different interaction surfaces with other SecY molecules inside the membrane. These interactions were significantly reduced when a translocating polypeptide inserted into the SecY channel co- or post-translationally. Mutations that abolish the interaction between SecY molecules still supported viability of E. coli. These results show that a single SecY molecule is sufficient for protein translocation.     

Vision: when the clock appears to stop

Eye movements produce a temporary loss of visual sensitivity known as saccadic suppression, and a distortion of space perception known as saccadic compression. A new study has reported a seemingly related phenomenon --chronostasis---in which one's perception of time also undergoes an illusory distortion during rapid movements of the eyes.     

Telomerase RNA template mutations reveal sequence-specific requirements for the activation and repression of telomerase action at telomeres

Telomeric DNA is maintained within a length range characteristic of an organism or cell type. Significant deviations outside this range are associated with altered telomere function. The yeast telomere-binding protein Rap1p negatively regulates telomere length. Telomere elongation is responsive to both the number of Rap1p molecules bound to a telomere and the Rap1p-centered DNA-protein complex at the extreme telomeric end. Previously, we showed that a specific trinucleotide substitution in the Saccharomyces cerevisiae telomerase gene (TLC1) RNA template abolished the enzymatic activity of telomerase, causing the same cell senescence and telomere shortening phenotypes as a complete tlc1 deletion. Here we analyze effects of six single- and double-base changes within these same three positions. All six mutant telomerases had in vitro enzymatic activity levels similar to the wild-type levels. The base changes predicted from the mutations all disrupted Rap1p binding in vitro to the corresponding duplex DNAs. However, they caused two classes of effects on telomere homeostasis: (i) rapid, RAD52-independent telomere lengthening and poor length regulation, whose severity correlated with the decrease in in vitro Rap1p binding affinity (this is consistent with loss of negative regulation of telomerase action at these telomeres; and (ii) telomere shortening that, depending on the template mutation, either established a new short telomere set length with normal cell growth or was progressive and led to cellular senescence. Hence, disrupting Rap1p binding at the telomeric terminus is not sufficient to deregulate telomere elongation. This provides further evidence that both positive and negative cis-acting regulators of telomerase act at telomeres.