The rpoS gene from Pseudomonas sp. M18, which encodes predicted protein (an alternative sigma factor s, sigma(S), or sigma(38)) with 99.5% sequence identity with RpoS from Pseudomonas aeruginosa PAO1, was first cloned. In order to investigate the mechanism of rpoS expression, an rpoS null mutant, named M18S, was constructed with insertion of aacC1 cassette bearing a gentamycin resistance gene. With introduction of a plasmid containing an rpoS'-'lacZ translational fusion (pMERS) to wild-type strain M18 or M18S, it was first found that beta-galactosidase activity expressed in strain M18S (pMERS) decreased to fourfold of that expressed in the strain M18 (pMERS). When strain M18S (pMERS) was introduced with another plasmid pBBS containing the wild-type rpoS gene, its beta-galactosidase expression level was enhanced and almost restored to that in strain M18 (pMERS). Similarly, expression of beta-galactosidase from a chromosomal fusion of the promoter of the wild-type rpoS gene with lacZ (rpoS-lacZ) was enhanced fivefold in the presence of a plasmid with the wild-type rpoS gene. With these findings, it is suggested that RpoS sigma factor may be involved in autoinducing its own gene expression in Pseudomonas sp. M18.