An inverted repeat structure from Bacillus natto preceding the Bacillus subtilis alpha-amylase gene has been suggested to be responsible for the enhancement of alpha-amylase production [Yamazaki et al. (1983) J. Bacteriol. 156, 327-337]. A similar inverted repeat region has also been found upstream from the Bacillus amyloliquefaciens alpha-amylase gene and shown to function as a transcription termination signal of an upstream operon of 2.2 X 10(3) bases (2.2 kb) (Kallio et al., following paper in this journal). The removal of this DNA region reduced the level of alpha-amylase production by 70% and led to concomitant formation of a readthrough transcript arising from the promoter of the 2.2-kb operon. To test whether the readthrough transcript affected the alpha-amylase production, the promoter of the 2.2-kb operon was removed from plasmid constructions carrying either intact or deleted inverted repeat sequences. When cultures carrying the above plasmids were assayed for alpha-amylase activity, both constructions produced equal amounts of alpha-amylase. Thus, the inverted repeat structure preceding the alpha-amylase promoter does not, as such, enhance the alpha-amylase production. Instead, its presence prevents the inhibition of alpha-amylase expression caused by the readthrough transcripts from the upstream promoter.