FUNCTIONAL-ANALYSIS OF BLOCK-5, ONE OF THE HIGHLY CONSERVED AMINO-ACID-SEQUENCES IN THE 130-KDA CRYIVA PROTEIN PRODUCED BY BACILLUS-THURINGIENSIS SUBSP ISRAELENSIS

There are five amino acid sequences highly conserved among Bacillus th uringiensis delta-endotoxins. We have changed the amino acid residues in block 5, one of the conserved sequences, of CryIVA When the amino a cid residues with charged side chains were replaced by others, the amo unt of production of the altered CryIVA protein was markedly decreased . It is suggested that the decrease is caused by the unstable conforma tion of the altered CryIVA protein molecule, as judged by digestion wi th trypsin and thermolysin. On the other hand, the substitution of ami no acid residues in block 5 did not affect the insecticidal activity o f CryIVA. These results strongly suggest that block 5 of CryIVA is one of the stability-determining elements of the protoxin molecule.