Meiotic recombination in budding yeast requires two RecA-related proteins, Rad51 and Dmc1, both of which form filaments on DNA capable of directing homology search and catalyzing formation of homologous joint molecules (JMs) and strand exchange. Using a separation-of-function mutant form of Rad51, that retains filament-forming but not JM forming activity, we show that the JM activity of Rad51 is fully dispensable for meiotic recombination. The corresponding mutation in Dmc1 causes a profound recombination defect, demonstrating Dmc1’s JM activity alone is responsible for meiotic recombination. We further provide biochemical evidence that Rad51 acts with Mei5-Sae3 as a Dmc1 accessory factor. Thus, Rad51 is a multifunctional protein that catalyzes recombination directly in mitosis and indirectly, via Dmc1, during meiosis.

Mammalian (rat) ribosomes have 80 proteins; the sequence of amino acids in 75 have been determined. What has been learned of the structure of the rat ribosomal proteins is reviewed with particular attention to their evolution and to amino acid sequence motifs. The latter include: clusters of basic or acidic residues; sequence repeats or shared sequences; zinc finger domains; bZIP elements; and nuclear localization signals. The occurrence and the possible significance of phosphorylated residues and of ubiquitin extensions is noted. The characteristics of the mRNAs that encode the proteins are summarized. The relationship of the rat ribosomal proteins to the proteins in ribosomes from humans, yeast, archaebacteria, and Escherichia coli is collated.

The structure of a 29-nucleotide RNA containing the sarcin͞ricin loop (SRL) of rat 28 S rRNA has been determined at 2.1 Å resolution. Recognition of the SRL by elongation factors and by the ribotoxins, sarcin and ricin, requires a nearly universal dodecamer sequence that folds into a G-bulged cross-strand A stack and a GAGA tetraloop. The juxtaposition of these two motifs forms a distorted hairpin structure that allows direct recognition of bases in both grooves as well as recognition of nonhelical backbone geometry and two 5-unstacked purines. Comparisons with other RNA crystal structures establish the cross-strand A stack and the GNRA tetraloop as defined and modular RNA structural elements. The conserved region at the top is connected to the base of the domain by a region presumed to be f lexible because of the sparsity of stabilizing contacts. Although the conformation of the SRL RNA previously determined by NMR spectroscopy is similar to the structure determined by x-ray crystallography, significant differences are observed in the ''f lexible'' region and to a lesser extent in the G-bulged cross-strand A stack.The highly conserved sarcin͞ricin loop (SRL), a component of 23-28 S rRNA, is essential for protein synthesis because it participates in the binding of elongation factors (EFs) to the ribosome. The function of this domain was illucidated by studying ribosomes treated with its namesake ribotoxins, sarcin and ricin (1, 2). The ribotoxins cleave a single covalent bond in the SRL RNA, which kill cells by inactivating ribosomes. Sarcin is a ribonuclease that cleaves the phosphodiester backbone on the 3Ј-side of G4325 (rat 28 S rRNA numbering is used throughout) (1, 2). Ricin depurinates the 5Ј-adjacent A4324 (3). EF-dependent functions are specifically impaired when ribosomes are treated with either toxin, whereas other ribosomal functions including EF-G-independent translocation are unaffected (1). Further indications of the domain's importance are that 12 of its 17 nucleotides are near universal and that chemical footprinting (4) marks it as the only rRNA region protected by both EFs.An SRL RNA can mimic the SRL in the ribosome because binding of EF-G to an Escherichia coli or rat SRL RNA is only Ϸ10-fold weaker than binding to intact E. coli ribosomes (5). Further, only the GTP-bound and apoprotein forms of EF-G bind the E. coli SRL RNA, whereas GDP-bound EF-G does not (5). EF-G binding is most affected by mutation of G4319 (5). Both toxins recognize and cleave a single bond in the SRL oligonucleotide (Fig. 1). Comprehensive mutational analysis of the SRL established that sarcin recognition and cleavage are largely dependent on a single base, the bulged G4319 (7). In contrast, each of the 4 nt of the GAGA tetraloop is necessary, and a GAGA tetraloop is sufficient for ricin recognition and depurination (6).The perception of RNA loop structure was altered by the determination of the conformation of the rat SRL by NMR spectroscopy (8, 9). Because the rules that govern formation of non-Watson-Crick ...

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Bulged-G motifs are ubiquitous internal RNA loops that provide specific recognition sites for proteins and RNAs. To establish the common and distinctive features of the motif we determined the structures of three variants and compared them with related structures. The variants are 27-nt mimics of the sarcin/ricin loop (SRL) from Escherichia coli 23S ribosomal RNA that is an essential part of the binding site for elongation factors (EFs). The wild-type SRL has now been determined at 1.04 A resolution, supplementing data obtained before at 1.11 A and allowing the first calculation of coordinate error for an RNA motif. The other two structures, having a viable (C2658U*G2663A) or a lethal mutation (C2658G*G2663C), were determined at 1.75 and 2.25 A resolution, respectively. Comparisons reveal that bulged-G motifs have a common hydration and geometry, with flexible junctions at flanking structural elements. Six conserved nucleotides preserve the fold of the motif; the remaining seven to nine vary in sequence and alter contacts in both grooves. Differences between accessible functional groups of the lethal mutation and those of the viable mutation and wild-type SRL may account for the impaired elongation factor binding to ribosomes with the C2658G*G2663C mutation and may underlie the lethal phenotype.