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The Escherichia coli endonuclease RNase E plays a key role in rRNA maturation and mRNA decay. In particular, it controls the decay of its own mRNA by cleaving it within the 5'-untranslated region (UTR), thereby autoregulating its synthesis. Here, we report that, when the synthesis of an RNase E substrate is artificially induced to high levels in vivo, both the rne mRNA concentration and RNase E synthesis increase abruptly and then decrease to a steady-state level that remains higher than in the absence of induction. Using rne-lacZ fusions that retain or lack the rne 5'UTR, we show that these variations reflect a transient mRNA stabilization mediated by the rne 5'UTR. Finally, by putting RNase E synthesis under the control of an IPTG-controlled promoter, we show that a similar, rne 5'UTR-mediated mRNA stabilization can result from a shortage of RNase E. We conclude that the burst in substrate synthesis has titrated RNase E, stabilizing the rne mRNA by protecting its 5'UTR. However, this stabilization is self-correcting, because it allows the RNase E pool to expand until its mRNA is destabilized again. Thus, autoregulation allows RNase E to adjust its synthesis to that of its substrates, a behaviour that may be common among autoregulated proteins. Incidentally, this adjustment cannot occur when translation is blocked, and we argue that the global mRNA stabilization observed under these conditions originates in part from this defect. 相似文献
3.
Glycosaminoglycans (GAG) are known to participate in central nervous system
processes such as development, cell migration, and neurite outgrowth, but
little is known with respect to their regulation through soluble
neurotrophic factors. In the present study, we have addressed this issue
using cell culture models of three distinct cell populations derived from
young rat retinas, namely, purified M uller glia, pigmented epithelium, and
neurons respectively. Cultures were maintained in chemically defined media
in the presence or absence of either basic fibroblast or epidermal growth
factor. In control glial and epithelial cultures, hyaluronic acid dominated
the soluble GAG pool, with lesser contributions from dermatan sulfate,
chondroitin sulfate, and heparan sulfate (in decreasing order). Retinal
neuronal GAG were almost exclusively chondroitin sulfate (approximately
90%). Treatment of glial and epithelial cultures with either factor led to
dose-dependent increases in especially hyaluronic acid synthesis (a maximum
6-fold increase relative to control levels), with smaller but consistent
changes in chondroitin sulfate. Similar treatment of retinal neurons did
not lead to any changes in GAG synthesis. These data indicate that glia and
pigment epithelia are the principal sources of GAG components in retina at
least in vitro, and that endogenous neurotrophic growth factors can greatly
modify GAG synthesis in these two retinal cell populations. Such data
suggest that a delicate balance may exist between growth factor
availability and glycoconjugate metabolism in vivo, participating in normal
or pathological states of the retina.
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The technique of gene fusion, in which the gene of interest, severed from its 3' end, is in-phase fused to a reporter gene--usually lacZ--is widely used to study translational regulation in Escherichia coli. Implicit in these approaches is the assumption that the activity of the ribosome binding site (RBS) fused in-phase with lacZ, does not per se modify the steady-state level of the lacZ mRNA. Herein, we have tested this hypothesis, using a model system in which the RBS of the lamB gene is fused to lacZ. Several point mutations affecting translation initiation have been formerly characterized in this RBS, and we used Northern blots to study their effect upon the lacZ mRNA pattern. Two series of constructs were assayed: in the first one, a 51-bp fragment centered around the lamB initiator codon, was inserted in front of lacZ within the natural lactose operon, whereas in the second the lacZ gene was fused to the genuine malK-lamB operon just downstream from the lamB RBS. We observed that in the first series, the concentration and average molecular weight of the lacZ mRNA dropped sharply as the efficiency of the RBS decreased. This apparently arose from a decreased stability of the message, since the mRNA patterns are equalized when the endonuclease RNase E is inactivated. We suggest that in this case the rate limiting step in the decay process is an RNase E cleavage that is outcompeted by translation.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
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Water-dependency appeared independently in several clades of the class Collembola, which is basically of terrestrial origin
according to recent phylogenetic analyses. Though moderately diversified (less than 8,000 species), Collembola are among the
most numerous terrestrial arthropods in wetland communities, with a small number of species living on the surface of water.
Many species are dependent on water-saturated atmosphere of caves, and on snow or ice in high mountains. A total of 525 water-dependent
species have been recognized, of which 103 are linked to free freshwaters and 109 to anchialine or marine waters. Many interstitial
species are also dependent to an unknown extent on water saturation in the deep layers of the soil. The numbers provided here
are underestimates, as Collembola are extremely poorly known outside the Holarctis, and the ecology of described species usually
not documented. However, a general biogeographical pattern is emerging from available data. The most remarkable feature is
that about 15% of the fauna is water-dependent in the holarctic region, compared to 4% in the tropics and southern hemisphere.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Guest editors: E. V. Balian, C. Lévêque, H. Segers & K. Martens
Freshwater Animal Diversity Assessment 相似文献