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Oligoribonuclease (Orn) is an essential 3'-to-5' hydrolytic exoribonuclease which degrades short oligoribonucleotides to 5' mono-nucleotides. Escherichia coli Orn has been crystallized under several different conditions using ammonium sulfate, sodium citrate and sodium acetate as precipitants. Both native and selenomethionine-labeled oligoribonuclease (SeMet-Orn) can be crystallized at room temperature in 1.4-1.55 M sodium citrate. The SeMet-Orn crystals diffract to 2.2 Å resolution and belong to space group P212121, with unit-cell parameters a = 70.43, b = 72.87, c = 147.76 Å, and two dimers in the asymmetric unit. When grown in the presence of manganese, a second crystal form (Mn-SeMet-Orn) was obtained containing a single dimer per asymmetric unit (P212121; a = 63.74, b = 74.31, c = 74.19 Å). Finally, a hexagonal crystal form was obtained using sodium acetate as a precipitant (a = 91.5, b = 91.5, c = 111.1 Å). This crystal (Zn-ApUp-Orn) belongs to the P65 space group and has three oligoribonuclease molecules per asymmetric unit. [ABSTRACT FROM AUTHOR]