A key step in cut-and-paste DNA transposition is the pairing of transposon ends before the element is excised and inserted at a new site in its host genome. Crystallographic analyses of the paired-end complex (PEC) formed from precleaved transposon ends and the transposase of the eukaryotic element Mos1 reveals two parallel ends bound to a dimeric enzyme. The complex has a trans arrangement, with each transposon end recognized by the DNA binding region of one transposase monomer and by the active site of the other monomer. Two additional DNA duplexes in the crystal indicate likely binding sites for flanking DNA. Biochemical data provide support for a model of the target capture complex and identify Arg186 to be critical for target binding. Mixing experiments indicate that a transposase dimer initiates first-strand cleavage and suggest a pathway for PEC formation.

Trypanosoma cruzi, the protozoan parasite that causes Chagas' disease in humans, has a complex life cycle alternating between the insect vector and the mammalian host. In the vector, it multiplies as noninfective epimastigotes that migrate to the hindgut and differentiate into infective metacyclic trypomastigotes. During the insect blood meal, the metacyclic trypomastigotes are deposited with the feces and urine near a skin wound, initiating the natural infection.T. cruzi is unable to synthesize sialic acids (SA), 1 but it expresses a unique trans-sialidase (TS), which transfers ␣2-3-linked SA from host glycoproteins and glycolipids to acceptors containing terminal ␤-galactosyl residues present on the parasite surface (reviewed in Refs. 1-4). Several studies characterizing the nature and structure of the SA acceptors have been published. These acceptors are abundant on the parasite surface and were first described as major surface glycoproteins of epimastigotes by Alves and Colli (5), who called them bands A, B, and C. Subsequently, a similar cell surface glycoprotein complex, called GP24, GP31, and GP37 was described by Ferguson et al. (6), and Previato et al. (7) first described a 43-kDa SA acceptor. More recently, they have been called 38/43 glycoconjugates (8), and the so called epimastigote lipophosphoglycan-like molecule could belong to the same family of molecules (9). In metacyclic trypomastigote forms, the SA acceptors were reported originally as the 35/50-kDa antigens (10, 11) that were subsequently defined as mucin-like glycoproteins (12). In the trypomastigote forms found in mammals, the SA acceptors were described as a group of molecules that share the stagespecific epitope 3 (Ssp-3) (13), an epitope dependent on parasite

No abstract

No abstract

We present the crystal structure of the catalytic domain of Mos1 transposase, a member of the Tc1/mariner family of transposases. The structure comprises an RNase H-like core, bringing together an aspartic acid triad to form the active site, capped by N-and C-terminal a-helices. We have solved structures with either one Mg 2 þ or two Mn 2 þ ions in the active site, consistent with a two-metal mechanism for catalysis. The lack of hairpin-stabilizing structural motifs is consistent with the absence of a hairpin intermediate in Mos1 excision. We have built a model for the DNA-binding domain of Mos1 transposase, based on the structure of the bipartite DNA-binding domain of Tc3 transposase. Combining this with the crystal structure of the catalytic domain provides a model for the paired-end complex formed between a dimer of Mos1 transposase and inverted repeat DNA. The implications for the mechanisms of first and second strand cleavage are discussed.