Iron-regulated haemolysin gene from Edwardsiella tarda

We have cloned and sequenced the haemolysin gene locus from Edwardsiella tarda (ETH). This region encoded two open reading frames, designated ethA and ethB. ethA is the haemolysin gene consisting of 4782 bp encoding a product of 165.3 kDa and ethB is an activation/secretion protein gene of 1677 bp that encodes a product of 61.9 kDa. There were two putative ferric uptake regulator (Fur) binding sites on the 5' upstream region of the ethB gene overlapping the promoter region and ribosome-binding site. The haemolysin produced by the cloned gene was secreted by Escherichia coli. The deduced amino acid sequences of the ethA and ethB genes were found to be homologous to those of the haemolysin and activation/secretion proteins of Haemophilus ducreyi, Proteus mirabilis, and Serratia marcescens. E. coli carrying the ethA gene but not the ethB gene completely lost haemolytic activity, although the ethA gene was transcribed. The protein expressed by E. coli carrying a recombinant plasmid which encoded the ethA gene had haemagglutination activity. The EthB protein was necessary for activation of EthA protein (haemolysin). The ethA and ethB genes were very prevalent in haemolytic E. tarda strains isolated from diseased fish. Transcription of the ethB gene was regulated by iron. The ethA and ethB genes were transcribed independently.