Abstract
The role of the three cysteine residues at positions 13, 63 and 133 in Escherichia coli RNAase H, an enzyme that is sensitive to N-ethylmaleimide [Berkower, Leis & Hurwitz (1973) J. Biol. Chem. 248, 5914-5921], was examined by using both site-directed mutagenesis and chemical modification. Novel aspects that were found are as follows. First, none of the cysteine residues is required for activity. Secondly, chemical modification of either Cys-13 or Cys-133 with thiol-blocking reagents inactivates the enzyme, but that of Cys-63 does not. Thus the sensitivity of E. coli RNAase H to N-ethylmaleimide arises not from blocking of the thiol group but from steric hindrance by the modifying group incorporated at either Cys-13 or Cys-133.