Agradecimentos: This work was supported in part by the Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP 01/07533-7, 05/03234-6 and 12/51580-4) and Conselho Nacional de Desenvolvimento Científico e Tecnologico (CNPq). ATPM received a M. Sc. fellowship from Coor- denação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). EMBF received a PhD fellowship from FAPESP (2011/15792-4). MAR received a PhD fellowship from CNPq (140377/2008-5) and a grant (PIQ) from IFSULDEMINAS. MAS and CAS were supported by graduate fellowships from FAPESP (2004/02540-3, 2008/55690-3 and 2006/52844-4). APS is the recipient of a research fellowship from CNPq. RA was the recipient of a CNPq research grant and a productivity fellowship. We gratefully acknowledge LNLS for beamline time Ver menos

Xylella fastidiosa is a xylem-limited bacterium that infects a wide variety of plants. Stationary phase survival protein E is classified as a nucleotidase, which is expressed when bacterial cells are in the stationary growth phase and subjected to environmental stresses. Here, we report four refined X-ray structures of this protein from X. fastidiosa in four different crystal forms in the presence and/or absence of the substrate 30-AMP. In all chains, the conserved loop verified in family members assumes a closed conformation in either condition. Therefore, the enzymatic mechanism for the target protein might be different of its homologs. Two crystal forms exhibit two monomers whereas the other two show four monomers in the asymmetric unit. While the biological unit has been characterized as a tetramer, differences of their sizes and symmetry are remarkable. Four conformers identified by SmallAngle X-ray Scattering (SAXS) in a ligand-free solution are related to the low frequency normal modes of the crystallographic structures associated with rigid body-like protomer arrangements responsible for the longitudinal and symmetric adjustments between tetramers. When the substrate is present in solution, only two conformers are selected. The most prominent conformer for each case is associated to a normal mode able to elongate the protein by moving apart two dimers. To our knowledge, this work was the first investigation based on the normal modes that analyzed the quaternary structure variability for an enzyme of the SurE family followed by crystallography and SAXS validation. The combined results raise new directions to study allosteric features of XfSurE protein Ver menos