Packing of monodisperse DNA-RecA protein complexes

We study E.coli RecA protein, from preparation and purification of protein monomers to structural studies of protein polymers formed on DNA – nucleoprotein filaments. RecA protein is a multirole one, where DNA strand exchange by forming nucleoprotein filament during homologous recombination and cleavage of SOS response repressors are the most prominent two. The first role, where we are interested into structural details (and structure makes function, very much so in this case) also came recently into focus with the work of Zahradka et al.[1], where reassembly of shattered chromosomes in D. radiodurans is completed by RecA-dependent crossovers. RecA is a relatively small protein, MW = 37, 842, with 352 amino acid residues. Only by polymerizing within nucleoprotein filaments it achieves its function. The RecA polymers have been crystallized and structure determined to atomic resolution by XRD [2], however the structure of RecA-DNA complex is not solved, and the exact path of DNA within the nucleoprotein filament is not known, although it has been extensively studied by SANS, electron microscopy or NMR [3]. We form RecA nucleoprotein filaments using very short, monodisperse, 146 bp long DNA [4]. Such 50~75 nm long filaments are shorter than their respective persistence length – i.e. they should behave as straight rods. A monodisperse colloidal system of helical rodlike particles is capable of forming liquid crystal [5]. This, indeed, might be the most ordered possible preparation of nucleoprotein filaments. Studying it by optical and electronic microscopies and XRD will allow further insight into the function of RecA. There are two routes of investigation: the exact structural parameters of the short filaments and the properties of the liquid crystalline phase they could form. [1] Zahradka K., Slade D., Bailone A., Sommer S., Averbeck D., Petranovic M., Lindner A.B. & Radman M., Nature 443: 569-573 (2006). [2] Story R. M., Weber I. T. & Steitz T. A., Nature 355: 318– 325 (1992). [3] DiCapua E., Schnarr M., Ruigrok R.B.W., Lindner P., Timmins P.A., J. Mol. Biol. 214: 557-570 (1990) ; Yu X., Jacobs S.A., West S.C., Ogawa T. & Egelman E.H., PNAS 98: 8419-8424 (2001) ; Nishinaka T., Shinohara A., Ito Y., Yokoyama S. & Shibata T., PNAS 95:11071-11076 (1998). [4] Sikorav J.-L., Pelta J. & Livolant F., Biophys. J. 67:1387– 1392 (1994). [5] Leforestier A. & Livolant F., Biophys. J. 65:56-72 (1993)

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