Prion nonsense

MONITOR

Bubbles, loops and globins Genetic dissection of a mammalian replicator in the human b-globin locus Aladjem, M. et al. Science 281, 1005–1009 Origins of DNA replication exist in mammalian cells but specific sequences required for replication initiation have not been identified until now. This validation of the replicon hypothesis in mammalian cells by Aladjem et al. was possible only by abandoning extrachromosomal replication assays in favour of a novel system that involves site-specific integration into the genome of CV-1 cells. Using these cells, replication initiation was shown to occur at two

locations within a core element of the b-globin replicator. By comparison with replication initiation in different b-globin fragments, it is clear that this function requires the core and at least one of two flanking auxiliary sequences. The power of this exciting new system is that specific DNA sequences and transacting factors that functionally participate in mammalian DNA replication can now be identified. Moreover, the timing of replication at the b-globin gene is

Prion nonsense A critical role for amino-terminal glutamine/asparagine repeats in the formation and propagation of a yeast prion DePace, A.H., Santoso, A., Hillner, P. and Weissman, J.S. Cell 93, 1241–1252 The yeast [PSI+] factor is a nonmendelian trait that causes suppression of nonsense mutations and is propagated by a prion-like mechanism. Nonsense suppression is mediated by conversion of the Sup35p translation termination factor from a soluble, functional state to non-functional insoluble amyloid fibrils. The N-terminal Sup35p PrD domain is both necessary and sufficient for conversion of Sup35p, or a heterologous protein such as GFP, to intracellular aggregates. Now, using a genetic

screen to identify Anti-SUppressor (ASU) and Psi No More (PNM) mutants, DePace et al. have identified PrD domain residues critical for amyloid formation and propagation. Mutant PrD domains, fused to GFP, result in significantly decreased recruitment into intracellular punctate aggregates in [PSI+] cells. Similarly, most of the ASU/PNM mutations lead to an increase in the amount of soluble Sup35p protein in vivo and a decrease in Sup35p conversion kinetics in vitro. Remarkably, all ASU/PNM mutations

bicoid outside the Dipterans? Regulation of the Tribolium homologues of caudal and hunchback in Drosophila: evidence for maternal gradient systems in a short germ embryo Wolf, C. et al. Development 125, 3645–3654 The rapid mode of embryonic development of Drosohpila is not necessarily representative for all insects. Therefore, the general nature of concepts obtained through fly genetics needs to be tested in related but distinct species. For instance, it has been argued that the anterior morphogen Bicoid (BCD) represents a recent addition specific to higher dipterans. This recent publication challenges this view by providing evidence for the existence of a bcd-like activity outside Dipterans. However, it also confirms the idea that the manner by which BCD governs segmentation in flies is not generally conserved. The Tribolium

homologue of the posterior determinant caudal is, like the fly gene, translationally regulated in a bcd-dependent manner when placed in Drosophila. This strongly suggests that a bcd-like activity exists in beetles, despite the failure to isolate a bcdhomologue in species other than Dipterans. However, until such a homologous gene is identified, it remains possible that this bcd-like activity is, as in Caenorhabditis, mediated by a gene non-homologous to bcd. Although the paper also shows that the Tribolium homologue of hunchback (hb), a bcd transcriptional target gene, is also regulated by bcd in TIG DECEMBER 1998 VOL. 14 NO. 12

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controlled by the locus control region (LCR) that has been extensively characterized for its influence on transcription. A prevailing model for LCR activation invokes a DNA looping mechanism between its component hypersensitive sites and the b-globin promoter. This very promoter is co-localized with the b-globin replicator core and both are controlled by the LCR. How would these putative DNA loops influence formation of the replication bubble on the b-globin replicator while also stimulating transcription? Experimental approaches to answer this fascinating question have suddenly become possible and are likely to illuminate the shadowy link between transcription and replication.✍

occur at the extreme N-terminus of the PrD domain and most affect glutamine or asparagine residues enriched in this region. The importance of these residues is elegantly confirmed by the demonstration that replacing this PrD region with polyglutamine allows Sup35p to form new aggregates and to be recruited into existing aggregates in vivo. This mechanism may have important implications for human disease. A number of heritable neurodegenerative conditions, such as Huntington disease, are caused by expansion of polyglutamine repeats, recently shown to lead to amyloid fibril formation in vivo and in vitro. This raises the exciting prospect that studying [PSI+] might uncover molecular mechanisms involved in prion and non-prion amyloid diseases. ✍

flies, it is clear that hb has undergone regulatory shifts between flies and beetles. In flies, the gap gene domain of hb is activated by bcd, whereas the corresponding domain in beetles is probably under caudal control. This suggests that a major event during the evolution of insects has involved a change in the regulation of the gap gene hb. The fly-specific BCD-control of hb might therefore be of very derived character, despite serving as a prime model for the activity of a morphogenetic gradient. ✍

Monitor contributors this month Howy Jacobs, James Ellis, Adele Rowley, Ernst A. Wimmer and Claude Desplan