Correlation of the insecticidal activity of the Bacillus thuringiensis A4 strain against Bactrocera oleae (Diptera) with the 140-kDa crystal polypeptide

The parasporal crystals of Bacillus pumilus strain 15.1: a potential virulence factor?

Bacillus pumilus strain 15.1 was previously found to cause larval mortality in the Med-fly Ceratitis capitata and was shown to produce crystals in association with the spore. As parasporal crystals are well-known as invertebrate-active toxins in entomopathogenic bacteria such as Bacillus thuringiensis (Cry and Cyt toxins) and Lysinibacillus sphaericus (Bin and Cry toxins), the B. pumilus crystals were characterized. The crystals were composed of a 45 kDa protein that was identified as an oxalate decarboxylase by peptide mass fingerprinting, N-terminal sequencing and by comparison with the genome sequence of strain 15.1. Synthesis of crystals by a plasmid-cured derivative of strain 15.1 (produced using a novel curing strategy), demonstrated that the oxalate decarboxylase was encoded chromosomally. Crystals spontaneously solubilized when kept at low temperatures, and the protein produced was resistant to trypsin treatment. The insoluble crystals produced by B. pumilus 15.1 did not show significant toxicity when bioassayed against C. capitata larvae, but once the OxdD protein was solubilized, an increase of toxicity was observed. We also demonstrate that the OxdD present in the crystals has oxalate decarboxylate activity as the formation of formate was detected, which suggests a possible mechanism for B. pumilus 15.1 activity. To our knowledge, the characterization of the B. pumilus crystals as oxalate decarboxylase is the first report of the natural production of parasporal inclusions of an enzyme.

Germinant generation from δ-endotoxin of Bacillus thuringiensis strain 1.1

The novel finding of this study is that the δ-endotoxin present in the spore coat of Bacillus thuringiensis strain 1.1 (Bt1.1), plays a central role in spore germination by generation of germinant via its β-glucosidase activity and is based on the following: (i) the crystals of Bt1.1 consist of the 140 kDa δ-endotoxin which exhibits β-glucosidase enzymatic activity. Besides crystals, δ-endotoxin is also located in the spore coat and at this site displays β-glucosidase activity, resulting in glucose production; (ii) glucose is an efficient germinant of both Bt1.1 and acrystalliferous Bt4.1 strain; (iii) substrates of β-glucosidase can activate the germination of Bt1.1 spores, but not those of the acrystalliferous Bt4.1 sister strain that do not contain the 140 kDa δ-endotoxin; (iv) Reduction or enhancement of enzymatic activity of δ-endotoxin, results in retardation or acceleration of germination and outgrowth, respectively. Bt1.1 cells secrete a 60 kDa polypeptide which displays β-glucosidase activity as indicated by zymogram analysis and which is immunologically related to the 140 kDa δ-endotoxin.