SUMMARY: Many insects have a cell-free immune system in which small basic proteins called cecropins are the main defence against Gram-negative bacteria. We have earlier shown that an insect pathogenic strain of was resistant to insect immunity and that certain mutants resistant to phage φJ become sensitive to cecropins. We have found that protease-deficient mutants with and without resistance to φJ appear to be deficient in the mechanism of protease induction. Three different protease fractions exist and for two of the enzymes we describe a partial purification and characterization. The proteases show pronounced autodegradation which increases with the purity. Both enzymes are only partly affected by EDTA and they are highly toxic to All three enzymes destroy cecropins in immune haemolymph from Cecropia pupae. However, experiments with different mutants indicate that in , passive resistance to insect immunity is more important for virulence in than the production of proteases which can destroy cecropins.


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