Constitutive expression of Pasteurella multocida toxin

The expression of the Pasteurella multocida toxin (PMT) gene toxA was investigated. Growth in vitro at 30 degrees C or added iron caused less than 4-fold repression of toxA expression. The putative repressor TxaR was expressed in Escherichia coli but deletion and frameshift mutations abolishing TxaR production had no effect on toxA expression. Naturally occurring non-toxigenic mutants which contained the toxA gene had no large rearrangements near toxA or changes in toxA promoter structure. Thus PMT is constitutively expressed and is only regulated in a minor way.