Crystal structure of the primary piRNA biogenesis factor Zucchini reveals similarity to the bacterial PLD endonuclease Nuc.
Voigt, F., Reuter, M., Kasaruho, A., Schulz, E.C., Pillai, R.S. & Barabas, O.
RNA. 2012 Dec;18(12):2128-34. doi: 10.1261/rna.034967.112. Epub 2012 Oct 19.
Piwi-interacting RNAs (piRNAs) are a gonad-specific class of small RNAs that associate with the Piwi clade of Argonaute proteins and play a key role in transposon silencing in animals. Since biogenesis of piRNAs is independent of the double-stranded RNA-processing enzyme Dicer, an alternative nuclease that can process single-stranded RNA transcripts has been long sought. A Phospholipase D-like protein, Zucchini, that is essential for piRNA processing has been proposed to be a nuclease acting in piRNA biogenesis. Here we describe the crystal structure of Zucchini from Drosophila melanogaster and show that it is very similar to the bacterial endonuclease, Nuc. The structure also reveals that homodimerization induces major conformational changes assembling the active site. The active site is situated on the dimer interface at the bottom of a narrow groove that can likely accommodate single-stranded nucleic acid substrates. Furthermore, biophysical analysis identifies protein segments essential for dimerization and provides insights into regulation of Zucchini's activity.
Resetting the site: redirecting integration of an insertion sequence in a predictable way.
Guynet, C., Achard, A., Hoang, B.T., Barabas, O., Hickman, A.B., Dyda, F. & Chandler, M.
Mol Cell. 2009 Jun 12;34(5):612-9.
Target site choice is a complex and poorly understood aspect of DNA transposition despite its importance in rational transposon-mediated gene delivery. Though most transposons choose target sites essentially randomly or with some slight sequence or structural preferences, insertion sequence IS608 from Helicobacter pylori, which transposes using single-stranded DNA, always inserts just 3' of a TTAC tetranucleotide. Our results from studies on the IS608 transposition mechanism demonstrated that the transposase recognizes its target site by co-opting an internal segment of transposon DNA and utilizes it for specific recognition of the target sites through base-pairing. This suggested a way to redirect IS608 transposition to novel target sites. As we demonstrate here, we can now direct insertions in a predictable way into a variety of different chosen target sequences, both in vitro and in vivo.
In vitro reconstitution of a single-stranded transposition mechanism of IS608.
Guynet, C., Hickman, A.B., Barabas, O., Dyda, F., Chandler, M. & Ton-Hoang, B.
Mol Cell. 2008 Feb 15;29(3):302-12.
Bacterial insertion sequences (IS) play an important role in restructuring their host genomes. IS608, from Helicobacter pylori, belongs to a newly recognized and widespread IS group with a unique transposition mechanism. We have reconstituted the entire set of transposition cleavage and strand transfer reactions in vitro and find that, unlike any other known transposition system, they strictly require single-strand DNA. TnpA, the shortest identified transposase, uses a nucleophilic tyrosine for these reactions. It recognizes and cleaves only the IS608 "top strand." The results support a transposition model involving excision of a single-strand circle with abutted left (LE) and right (RE) IS ends. Insertion occurs site specifically 3' to conserved and essential TTAC tetranucleotide and appears to be driven by LE. This single-strand transposition mode has important implications not only for dispersion of IS608 but also for the other members of this very large IS family.
Mechanism of IS200/IS605 family DNA transposases: activation and transposon-directed target site selection.
Barabas, O., Ronning, D.R., Guynet, C., Hickman, A.B., Ton-Hoang, B., Chandler, M. & Dyda, F.
Cell. 2008 Jan 25;132(2):208-20.
The smallest known DNA transposases are those from the IS200/IS605 family. Here we show how the interplay of protein and DNA activates TnpA, the Helicobacter pylori IS608 transposase, for catalysis. First, transposon end binding causes a conformational change that aligns catalytically important protein residues within the active site. Subsequent precise cleavage at the left and right ends, the steps that liberate the transposon from its donor site, does not involve a site-specific DNA-binding domain. Rather, cleavage site recognition occurs by complementary base pairing with a TnpA-bound subterminal transposon DNA segment. Thus, the enzyme active site is constructed from elements of both protein and DNA, reminiscent of the interdependence of protein and RNA in the ribosome. Our structural results explain why the transposon ends are asymmetric and how the transposon selects a target site for integration, and they allow us to propose a molecular model for the entire transposition reaction.