The elastase structural gene from Pseudomonas aeruginosa IFO 3455 has been cloned and sequenced. Using this gene as a probe, we cloned the DNA fragments (pEL3080R, pEL10, and pEL103R) of the elastase gene from non-elastase-producing strains (P. aeruginosa IFO 3080, N-10, and PA103 respectively). These three Pseudomonas strains showed no detectable levels of elastase antigenicity by Western blotting (immunoblotting) or by elastase activity. When elastase structural genes about 8 kb in length were cloned into pUC18, an Escherichia coli expression vector, we were able to detect both elastase antigenicity and elastolytic activity in two bacterial clones (E. coli pEL10 and E. coli pEL103R). However, neither elastolytic activity nor elastase antigenicity was detected in the E. coli pEL308OR clone, although elastase mRNA was observed. The partial restriction map determined with several restriction enzymes of these three structural genes corresponded to that of P. aeruginosa ff0 3455. We sequenced the three DNA segments of the elastase gene from non-elastase-producing strains and compared the sequences with those from the elastase-producing P. aeruginosa strains IFO 3455 and PAO1. In P. aeruginosa N-10 and PA103, the sequences were almost identical to those from elastase-producing strains, except for several nucleotide differences. These minor differences may reflect a microheterogeneity of the elastase gene. These results suggest that two of the non-elastase-producing strains have the normal elastase structural gene and that elastase production is repressed by regulation of this gene expression in P. aeruginosa. Possible reasons for the lack of expression in these two strains are offered in this paper. In P. aeruginosa IFO 3080, the sequence had a 1-base deletion in the coding region, which should have caused a frameshift variation in the amino acid sequence. At present, we have no explanation for the abnormal posttranscriptional behavior of this strain.Pseudomonas aeruginosa is an opportunistic pathogen which can cause fatal infections in vulnerable hosts (19). Several components related to its virulence have been identified and characterized (22). Among these, elastase acts to inactivate a variety of biologically important proteins or phenomena (12,14,(16)(17)(18)23). In addition, there is strong evidence that elastase plays an important role in the pathogenesis of P. aeruginosa infections (11,20). Although the regulatory mechanism of elastase production is still unknown, elastase-deficient Pseudomonas strains have been isolated (18). Furthermore, some elastase-positive Pseudomonas isolates often become elastase negative after several passages in vitro.We (6, 29) and another group (2, 25) have independently cloned and sequenced the elastase structural gene using the elastase-producing P. aeruginosa strains IFO 3455 and PA01, respectively. Several elastase-related products (8, 9, 21), the existence of "pro" protein (2,6,9,29), and elastase activation molecules (9, 26) have been described. In addition, Goldberg and Ohm...