Mosquito larvicidal and hemolytic proteins purified from parasporal inclusions produced by Bacillus thuringiensis strain PG-14 (serotype 8a:8b)

Characterization of a mosquitocidal Bacillus thuringiensis serovar sotto strain isolated from Okinawa, Japan

AIMS

To characterize the mosquitocidal activity of parasporal inclusions of the Bacillus thuringiensis serovar sotto strain 96-OK-85-24, for comparison with two well-characterized mosquitocidal strains.

METHODS AND RESULTS

The strain 96-OK-85-24 significantly differed from the existing mosquitocidal B. thuringiensis strains in: (1) lacking the larvicidal activity against Culex pipiens molestus and haemolytic activity, and (2) SDS-PAGE profiles, immunological properties and N-terminal amino acid sequences of parasporal inclusion proteins.

CONCLUSIONS

It is clear from the results that the strain 96-OK-85-24 synthesizes a novel mosquitocidal Cry protein with a unique toxicity spectrum.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of the occurrence of a mosquitocidal B. thuringiensis strain with an unusual toxicity spectrum, lacking the activity against the culicine mosquito.

Characterization of mosquitocidal activity of Bacillus thuringiensis subsp. fukuokaensis crystal proteins

The mosquitocidal crystals of Bacillus thuringiensis subsp. fukuokaensis were isolated and bioassayed against fourth-instar larvae of two mosquito species. The 50% lethal concentration values of the crystals to Aedes aegypti and Culex quinquefasciatus were 4.1 and 2.9 micrograms/ml, respectively. In addition, the solubilized crystals had hemolytic activity; 50 micrograms/ml was the lowest detectable level. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that the crystals consisted of polypeptides of 90, 86, 82, 72, 50, 48, 37, and 27 kDa. When the solubilized inclusion was treated with C. quinquefasciatus midgut brush border membrane vesicles or Manduca sexta gut juice, only one major protein was detected. This protein retained mosquitocidal activity but had no detectable hemolytic activity. Immunological analysis of this subspecies and the subspecies israelensis, kyushuensis and darmstadiensis by using polyclonal antisera raised against the whole-crystal protein of B. thuringiensis subsp. fukuokaensis revealed that the proteins in subsp. fukuokaensis are distinct from proteins in the other subspecies because little cross-reaction was observed. Analysis of the plasmid pattern showed that the crystal protein genes are located on a plasmid of 130 MDa. Analysis of plasmid and chromosomal DNA from subsp. fukuokaensis showed little homology to the 72-kDa toxin gene (PG-14) of B. thuringiensis subsp. morrisoni. However, some of the proteins of B. thuringiensis subsp. fukuokaensis are homologous to other B. thuringiensis toxins because N-terminal amino acid analysis revealed that the 90-kDa protein is encoded by a cryIV gene type.

Affinity purification of a 65-kilodalton parasporal protein from Bacillus thuringiensis PG-14 that shows mosquitocidal activity

By using antibody-mediated affinity chromatography, a highly mosquito larvicidal but nonhemolytic fraction was obtained from alkali-solubilized, silkworm (Bombyx mori) larval gut juice-treated parasporal inclusions of Bacillus thuringiensis strain PG-14 (serotype 8a:8b). This fraction contained a 65-kDa protein only but not a 25-kDa protein, the main component in the flow through fraction unbound to the affinity column. The 25-kDa protein purified from the unbound fraction by CM-cellulose chromatography demonstrated a high hemolytic activity against sheep red blood cells but very low mosquito larvicidal activity.