haemolysin A (HlyA) is an exotoxin belonging to the Gram-negative bacterial RTX toxin family found in Escherichia coli. It is an important virulence factor for extra-intestinal infection and can be associated with increased severity of infection.
Synthesis and export of HlyA is directed by a 4 gene operon (hly), shown by classical transposon mutagenesis and then complementation. 2 genes are needed for synthesis and 2 for secretion, confirmed by DNA sequencing and protein expression. An outer membrane protein (TolC) encoded elsewhere is also required for secretion.
HlyA is unrelated to haemolysin E.