The extracellular elastase (33 kDa) of Pseudomonas aeruginosa is synthesized as a 53.6-kDa preproenzyme containing a long N-terminal propeptide, which is processed to the mature form via a 51-kDa proelastase. A 51-kDa protein isolated from Escherichia coli transformant carrying the Glu(141)-->Gln mutant elastase gene was subjected to N-terminal amino acid sequence analysis. No autoproteolytic processing of proelastase was expected to occur in these cells. The data indicated that the N-terminal sequence corresponds to the position between -174 and -164 of the preproelastase (numbers are in reference to the first amino acid residue of mature elastase). This confirms that the 51-kDa protein is proelastase and that the signal peptidase cleaves between Ala(-175) and Ala(-174).