Crystal structure of a new RNA‐binding domain from the antiterminator protein SacY of Bacillus subtilis

SacY belongs to a family of, at present, seven bacterial transcriptional antiterminators. The RNA‐binding and antitermination capacity of SacY resides in the 55 amino acids at the N‐terminal [SacY(1‐55)]. The crystal structure at 2 Å resolution shows that SacY(1‐55) forms a dimer in the crystal, in accordance with the NMR solution structure. The structure of the monomer is a four‐stranded β‐sheet with a simple β1β2β3β4 topology. One side of the sheet is covered by a long surface loop and the other side forms the dimer interface. The dimer is stabilized by the orthogonal stacking of the two β‐sheets. The crystal structure is in excellent agreement with the NMR solution structure (r.m.s. distance for Cα coordinates is 1.3 Å). The structure of SacY(1‐55) reveals a new RNA‐binding motif.