Dissecting Nucleases into Their Structural and Functional Domains: Mapping the RNA-Binding Surface o

The enzyme RNase III catalyzes the hydrolysis of the phosphodiester bond between two nucleotides. The protein was discovered in 1968, when Robertson and coworkers isolated a double-stranded RNA (dsRNA) cleaving activity in Escherichia coli cells (1 ). A few years later, specific targets for this enzyme were identified (2 ). Proteins analogous to RNase III have been isolated in many organisms, both eubacteria (3 ,4 ) and eukaryotes (5 ,6 ) suggesting the importance of the regulatory function of RNase III. The amount of RNase III present in the cell is downregulated by specific cleavage of its own messenger RNA. Recombinant RNase III is a dimer under physiological conditions (7 –11 ).