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The nucleotide sequence of a 2510 base pair chromosomal fragment containing the aerolysin gene aerA, and its regulatory region aerC, from a clinical isolate of Aeromonas sobria was determined. The aerolysin gene coded for a 54.5 kD polypeptide and had a G + C content of 59%, indicating that it is endogenous to the genus Aeromonas. In contrast, the aerC region was characterized by its high A + T content (61%) and the presence of a core motif, aATAAAa, repeated eight times within 300 base pairs. A 12 base pair repeat, 5′ AATAAAACCGGG3′ present within this region occurred as a direct repeat 544 base pairs away, within the coding region of aerolysin. RNA polymerase binding studies and S1 mapping allowed the detection of two divergent non-overlapping promoters within aerC. Despite having identical transcriptional start sites in both A. sobria and Escherichia coli, the amount of aerolysin transcript produced in E. coli is 30–40 times less than that found in A. sobria. The signal peptide of preproaerolysin was shown by deletion to be essential for export of the toxin to the external medium. The mature toxin is a hydrophilic protein with no hydrophobic stretches long enough to cross a membrane. A search for similarities to the primary sequence of aerolysin revealed that the toxin may share a functional similarity to haemolysin (hlyA) of E. coli. [ABSTRACT FROM AUTHOR]