CONSTRUCTION OF A DOUBLE HHA HNS MUTANT OF ESCHERICHIA-COLI - EFFECT ON DNA SUPERCOILING AND ALPHA-HEMOLYSIN PRODUCTION

A double hha hns Escherichia coli mutant was constructed. The effect o f the single hns mutation and of the double hha hns mutation on the ex pression of the cr haemolysin determinant of plasmid pANN202-312 was a ssessed. Whereas the hits mutant moderately increased expression of th e toxin, the double hha hns mutant strongly enhanced transcription of the hly operon and hence expression of the toxin. This suggests that b oth Hha and H-NS proteins participate in the modulation of the express ion of the toxin. The enhancement of haemolysin expression in the doub le mutant could not be correlated to a global alteration of DNA topolo gy: DNA preparations of a reporter plasmid isolated from this mutant g ave a topoisomer distribution similar to that of the parental strain.