Escherichia coil signal peptidase I (leader peptidase, SPase I) is an integral membrane serine protease that catalyzes the cleavage of signal (leader) peptides from pre-forms of membrane or secretory proteins. We previously demonstrated that E. coil SPase I was significantly inactivated by reaction with phenylglyoxal with concomitant modification of three to four of the total 17 arginine residues in the enzyme. This result indicated that several arginine residues are important for the optimal activity of the enzyme. In the present study, we have constructed 17 mutants of the enzyme by site-directed mutagenesis to investigate the role of individual arginine residues in the enzyme. Mutation of Arg127, Arg146, Arg198, Arg199, Arg226, Arg236, Arg275, Arg282, and Arg295 scarcely affected the enzyme activity in vivo and in vitro. However, the enzymatic activity toward a synthetic substrate was significantly decreased by replacements of Arg77, Arg222, Arg315, or Arg318 with alanine/lysine. The kcat values of the R77A, R77K, R222A, R222K, R315A, R318A, and R318K mutant enzymes were about 5.5-fold smaller than that of the wild-type enzyme, whereas the Km values of these mutant enzymes were almost identical with that of the wild-type. Moreover, the complementing abilities in E. Arg222, Arg315, coil IT41 were lost completely when Arg77, or Arg318 was replaced with alanine/lysine. The circular dichroism spectra and other enzymatic properties of these mutants were comparable to those of the wild-type enzyme, indicating no global conformational changes. However, the thermostability of R222A, R222K, R315A, and R318K was significantly lower compared to the wild type. Therefore, Arg77, Arg222, Arg315, and Arg318 are thought to be important for maintaining the proper and stable conformation of SPase I.