The three-dimensional structure of the 67K amino-terminal fragment of Escherichia coli DNA topoisomerase I has been determined to 2.2 A resolution. The polypeptide folds in an unusual way to give four distinct domains enclosing a hole large enough to accommodate a double-stranded DNA. The active-site tyrosyl residue, which is involved in the transient breakage of a DNA strand and the formation of a covalent enzyme-DNA intermediate, is present at the interface of two domains. The structure suggests a plausible mechanism by which E. coli DNA topoisomerase I and other members of the same DNA topoisomerase subfamily could catalyse the passage of one DNA strand through a transient break in another strand.

Post-translational modification of proteins by small polypeptides, such as ubiquitin, has emerged as a common and important mechanism for regulating protein function. Small ubiquitin-like modifier (SUMO) is a small protein that is structurally related to but functionally different from ubiquitin. We report the identification and functional analysis of AtSUMO1, AtSUMO2, and AtSCE1a as components of the SUMO conjugation (sumoylation) pathway in Arabidopsis. In yeast-two hybrid assays, AtSUMO1/2 interacts specifically with a SUMO-conjugating enzyme but not with a ubiquitin-conjugating enzyme. AtSCE1a, the Arabidopsis SUMO-conjugating enzyme ortholog, conjugates SUMO to RanGAP in vitro. AtSUMO1/2 and AtSCE1a colocalize at the nucleus, and AtSUMO1/2 are conjugated to endogenous SUMO targets in vivo. Analysis of transgenic plants showed that overexpression of AtSUMO1/2 does not have any obvious effect in general plant development, but increased sumoylation levels attenuate abscisic acid (ABA)-mediated growth inhibition and amplify the induction of ABA-and stress-responsive genes such as RD29A . Reduction of AtSCE1a expression levels accentuates ABA-mediated growth inhibition. Our results suggest a role for SUMO in the modulation of the ABA signal transduction pathway.

RNA triphosphatase is an essential mRNA processing enzyme that catalyzes the first step in cap formation. The 2.05 A crystal structure of yeast RNA triphosphatase Cet1p reveals a novel active site fold whereby an eight-stranded beta barrel forms a topologically closed triphosphate tunnel. Interactions of a sulfate in the center of the tunnel with a divalent cation and basic amino acids projecting into the tunnel suggest a catalytic mechanism that is supported by mutational data. Discrete surface domains mediate Cet1p homodimerization and Cet1p binding to the guanylyltransferase component of the capping apparatus. The structure and mechanism of fungal RNA triphosphatases are completely different from those of mammalian mRNA capping enzymes. Hence, RNA triphosphatase presents an ideal target for structure-based antifungal drug discovery.

Mechanisms regulating mammalian meiotic progression are poorly understood. Here we identify mouse YTHDC2 as a critical component. A screen yielded a sterile mutant, "ketu", caused by a Ythdc2 missense mutation. Mutant germ cells enter meiosis but proceed prematurely to aberrant metaphase and apoptosis, and display defects in transitioning from spermatogonial to meiotic gene expression programs. ketu phenocopies mutants lacking MEIOC, a YTHDC2 partner. Consistent with roles in post-transcriptional regulation, YTHDC2 is cytoplasmic, has 3ʹ→5ʹ RNA helicase activity in vitro, and has similarity within its YTH domain to an N 6 -methyladenosine recognition pocket. Orthologs are present throughout metazoans, but are diverged in nematodes and, more dramatically, Drosophilidae, where Bgcn is descended from a Ythdc2 gene duplication. We also uncover similarity between MEIOC and Bam, a Bgcn partner unique to schizophoran flies. We propose that regulation of gene expression by YTHDC2-MEIOC is an evolutionarily ancient strategy for controlling the germline transition into meiosis. Version 2: The following experimental changes were incorporated:• RNA-seq analyses of wild-type and Ythdc2 mutant testes at 8, 9, and 10 dpp (i.e., during the germline transition into meiosis).• Purification and characterization of recombinant YTHDC2 protein, demonstrating RNA helicase activity and effect of the ketu mutation.• Expanded histological and cytological analyses (DMC1-staining; PAS-staining and TUNEL assay at 8 dpp; further pH3 and tubulin staining along with SYCP3 staining in abnormal metaphases to evaluate premature metaphase entry; BrdU incorporation to evaluate premeiotic DNA replication; and more complete evaluation of persistent CCNA2 expression), including quantification of stainings at multiple ages (pH3, α-tubulin, BrdU, SYCP3, and CCNA2 stainings as well as TUNEL assay). Cem Meydan, Nathalie Lailler, Christopher E. Mason, and Christopher D. Lima were added as coauthors.