Functional recognition of the 3' splice site AG by the splicing factor U2AF35

In metazoans, spliceosome assembly is initiated through recognition of the 5' splice site by U1 snRNP and the polypyrimidine tract by the U2 small nuclear ribonucleoprotein particle (snRNP) auxiliary factor, U2AF. U2AF is a heterodimer comprising a large subunit, U2AF65, and a small subunit, U2AF35. U2AF65 directly contacts the polypyrimidine tract and is required for splicing in vitro. In comparison, the role of U2AF35 has been puzzling: U2AF35 is highly conserved and is required for viability, but can be dispensed with for splicing in vitro. Here we use site-specific crosslinking to show that very early during spliceosome assembly U2AF35 directly contacts the 3' splice site. Mutational analysis and in vitro genetic selection indicate that U2AF35 has a sequence-specific RNA-binding activity that recognizes the 3'-splice-site consensus, AG/G. We show that for introns with weak polypyrimidine tracts, the U2AF35-3'-splice-site interaction is critical for U2AF binding and splicing. Our results demonstrate a new biochemical activity of U2AF35, identify the factor that initially recognizes the 3' splice site, and explain why the AG dinucleotide is required for the first step of splicing for some but not all introns.     

Msh2 deficiency prevents in vivo somatic instability of the CAG repeat in Huntington disease transgenic mice

Huntington disease (HD), an autosomal dominant, progressive neurodegenerative disorder, is caused by an expanded CAG repeat sequence leading to an increase in the number of glutamine residues in the encoded protein. The normal CAG repeat range is 5-36, whereas 38 or more repeats are found in the diseased state; the severity of disease is roughly proportional to the number of CAG repeats. HD shows anticipation, in which subsequent generations display earlier disease onsets due to intergenerational repeat expansion. For longer repeat lengths, somatic instability of the repeat size has been observed both in human cases at autopsy and in transgenic mouse models containing either a genomic fragment of human HD exon 1 (ref. 9) or an expanded repeat inserted into the endogenous mouse gene Hdh (ref. 10). With increasing repeat number, the protein changes conformation and becomes increasingly prone to aggregation, suggesting important functional correlations between repeat length and pathology. Because dinucleotide repeat instability is known to increase when the mismatch repair enzyme MSH2 is missing, we examined instability of the HD CAG repeat by crossing transgenic mice carrying exon 1 of human HD (ref. 16) with Msh2-/- mice. Our results show that Msh2 is required for somatic instability of the CAG repeat.     

Childbearing outside marriage in western Europe

Across most European states in recent decades there have been significant increases in childbearing outside marriage. This article examines the extent to which women have their first child in one of four settings: prior to any partnership; in their first cohabiting partnership; in first marriage; and after a first partnership. Temporal changes in these behaviours and variation according to background characteristics of the women are also examined. For the women who had a child outside any partnership, we examine the extent to which they go on to form partnerships and how long after the birth this happens. For those who had their first child within a cohabiting union, we examine the extent to which they marry and how long after the birth this occurs. Finally, we investigate whether children born within cohabiting unions that do and do not convert into marriages are more or less likely than those born within marriage to see their parents separate.     

Which authorities are alike?

This article describes the first results from the recent revision to The ONS classification of local and health authorities of Great Britain to allow for boundary changes. For each local and health authority of Great Britain as at April 1999 it presents 'most similar' authorities as measured by a selection of 37 socioeconomic and demographic variables from the 1991 Census. These results will assist local and health authorities in identifying other appropriate authorities for comparative studies. A second article, to be published in Population Trends 99, will present the results of the analysis to identify groups of authorities in a hierarchy of new Clusters, Groups and Families for the revised classification.     

Three-dimensional structure of motor molecules

Images, calculated from electron micrographs, show the three-dimensional structures of microtubules and tubulin sheets decorated stoichiometrically with motor protein molecules. Dimeric motor domains (heads) of kinesin and ncd, the kinesin-related protein that moves in the reverse direction, each appeared to bind to tubulin in the same way, by one of their two heads. The second heads show an interesting difference in position that seems to be related to the directions of movement of the two motors. X-ray crystallographic results showing the structures of kinesin and ncd to be very similar at atomic resolution, and homologous also to myosin, suggest that the two motor families may use mechanisms that have much in common. Nevertheless, myosins and kinesins differ kinetically. Also, whereas conformational changes in the myosin catalytic domain are amplified by a long lever arm that connects it to the stalk domain, kinesin and ncd do not appear to possess a structure with a similar function but may rely on biased diffusion in order to move along microtubules.