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The ␤ protein of bacteriophage acts in homologous genetic recombination by catalyzing the annealing of complementary single-stranded DNA produced by the exonuclease. It has been shown that the ␤ protein binds to the products of the annealing reaction more tightly than to the initial substrates. We find that ␤ protein exists in three structural states. In the absence of DNA, ␤ protein forms inactive rings with Ϸ12 subunits. The active form of the ␤ protein in the presence of oligonucleotides or single-stranded DNA is a ring, composed of Ϸ15-18 subunits. The doublestranded products of the annealing reaction catalyzed by the rings are bound by ␤ protein in a left-handed helical structure, which protects the products from nucleolytic degradation. These observations suggest structural homology for a family of proteins, including the phage P22 erf, the bacterial RecT, and the eukaryotic Rad52 proteins, all of which are involved in homologous recombination.

Aim To examine species and trait composition in stream diatoms along environmental, climatic and spatial gradients and to ascertain if the use of different levels of biological organization is beneficial for investigating global environmental changes and the role of history in structuring communities. Location Global cover with datasets from the Antilles, France, Finland, New Zealand, La Réunion and the United States. Methods We related diatom species composition, guild composition, total richness and richness across guilds to environmental, climatic and spatial variables. We used non‐metric multidimensional scaling (NMDS) with environmental variable fitting, redundancy analysis (RDA) with variation partitioning, analysis of similarities and linear mixed models as statistical tools. Results Species composition differed significantly among the study regions, while the differences in guild composition were less pronounced. US and French streams shared a large number of species, whereas islands shared only a few species with continents. For species composition, all predictors showed significant relationships with diatoms but pH, longitude, annual temperature and precipitation had the strongest impact. Variation partitioning revealed that the local environment outperformed climatic and spatial variables. For guild composition, there was a substantial overlap across regions in NMDS. The results from RDA demonstrated, however, that guild composition was better explained than species composition, especially by environmental variables. Both species and guild richness were significantly correlated with most predictors. Notably, species richness scaled positively with latitude. Main conclusions Diatom species and guild composition varied substantially in response to local environment and climatic and spatial variables indicating both environmental and historical effects. Species composition discriminated the geographical regions better, while guild composition detected the environmental gradients better. This emphasizes the need to examine different levels of organization to gain a deeper understanding of the roles of environment versus history in structuring communities. These findings suggest that diatom species distributions are under strong microevolutionary constraints. Conversely, guild distributions are less dependent on historic factors and are driven primarily by the environment, which makes them better suited for research on global environmental change.

The bacterial RecA protein has been the most intensively studied enzyme in homologous genetic recombination. The core of RecA is structurally homologous to that of the F1-ATPase and helicases. Like the F1-ATPase and ring helicases, RecA forms a hexameric ring. The human Dmc1 (hDmc1) protein, a meiosis-specific recombinase, is homologous to RecA. We show that hDmc1 forms octameric rings. Unlike RecA and Rad51, however, hDmc1 protein does not form helical filaments. The hDmc1 ring binds DNA in the central channel, as do the ring helicases, which is likely to represent the active form of the protein. These observations indicate that the conservation of the RecA-like ring structure extends from bacteria to humans, and that some RecA homologs may form both rings and filaments, whereas others may function only as rings.The Escherichia coli RecA protein has been intensively studied for many years by using genetic, biochemical, and biophysical techniques (1-4). It has been hoped that insight gained into how RecA functions to catalyze homologous genetic recombination might be helpful in understanding such eukaryotic processes as meiosis. The RecA protein forms an unusual nucleoprotein filament on DNA, in which the DNA is both extensively stretched and untwisted (5-7). The yeast (8) and human (9) Rad51 proteins, which are homologous to RecA, form very similar nucleoprotein structures. The more distantly related bacteriophage T4 UvsX protein assembles in a filament with nearly identical helical parameters (10), suggesting that the structural properties of this filament are important to function and have been conserved over large evolutionary distances.RAD51 in yeast is involved in DNA recombination and repair (11), but is not an essential gene. However, disruption of RAD51 in mice leads to embryonic lethality (12), suggestive of a role in general DNA metabolism and genome stability. Another eukaryotic RecA homolog is the Dmc1 protein. DMC1 (disrupted meiotic cDNA) is a meiosis-specific gene in yeast and animals, and disruption of it leads to meiotic arrest in both yeast (13) and mice (14), and accumulation of doublestrand breaks with 3Ј ends (12). In at least one plant, Arabidopsis, however, Dmc1 is induced during mitosis (15). It has been shown in vitro that yeast (16) and human (17, 18) Rad51 proteins can promote an ATP-dependent strand-exchange reaction. Human Dmc1 (hDmc1) protein also can catalyze such reactions in vitro, but with much lower activity (19).The homology between RecA and its eukaryotic homologs is contained within the nucleotide-binding core. The nucleotide-binding core of the hexameric F1-ATPase is structurally homologous to the core of the RecA protein (20), even though these proteins have vastly different functions and substrates (the loops of RecA that bind DNA are topologically similar to the loops in the F1-ATPase that bind the ␥-subunit, a coiledcoil protein). In addition, the conserved motifs present in DNA and RNA helicases have been shown to be part of the same core found in RecA for thr...