Investigation of possible homologies between crystal proteins of three mosquitocidal strains ofBacillus thuringiensis

The Bacillus thuringiensis cyt genes for hemolytic endotoxins constitute a gene family

In the same way that cry genes, coding for larvicidal delta endotoxins, constitute a large and diverse gene family, the cyt genes for hemolytic toxins seem to compose another set of highly related genes in Bacillus thuringiensis. Although the occurrence of Cyt hemolytic factors in B. thuringiensis has been typically associated with mosquitocidal strains, we have recently shown that cyt genes are also present in strains with different pathotypes; this is the case for the morrisoni subspecies, which includes strains biologically active against dipteran, lepidopteran, and coleopteran larvae. In addition, while one Cyt type of protein has been described in all of the mosquitocidal strains studied so far, the present study confirms that at least two Cyt toxins coexist in the more toxic antidipteran strains, such as B. thuringiensis subsp. israelensis and subsp. morrisoni PG14, and that this could also be the case for many others. In fact, PCR screening and Western blot analysis of 50 B. thuringiensis strains revealed that cyt2-related genes are present in all strains with known antidipteran activity, as well as in some others with different or unknown host ranges. Partial DNA sequences for several of these genes were determined, and protein sequence alignments revealed a high degree of conservation of the structural domains. These findings point to an important biological role for Cyt toxins in the final in vivo toxic activity of many B. thuringiensis strains.

A broad-spectrum cytolytic toxin fromBacillus thuringiensisvar.kyushuensis

Bacillus thuringiensis (Bt) var. kyushuensis synthesizes a mosquitocidal crystalline inclusion containing several proteins ranging from 140 to 14 kDa. We have identified a 25 kDa protein protoxin in this inclusion which is not cytolytic, but when activated proteolytically to 23-22 kDa products is cytolytic to mosquito, lepidopteran and mammalian cells, can release entrapped glucose from liposomes and forms cation-selective channels in a planar lipid bilayer. This broad-spectrum cytolytic toxin is related antigenically to the 23 kDa toxin from Bt var. darmstadiensis strain 73-E10-2, but not to the 25 kDa CytA toxin of Bt var. israelensis. The cytolytic activity of these Bt var. kyushuensis toxins, like that of the latter two toxins, can be neutralized by incubation with liposomes containing phospholipids.

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.

Characterization of plasmids from Listeria sp

Out of 139 isolates of Listeria sp. (mainly L. monocytogenes) 107 (78%) contained extrachromosomal DNA. Plasmids from 51 of these isolates were investigated further and covered a range of 8 different-sized molecules as shown by agarose gel electrophoresis. Only one of the 107 isolates contained more than one plasmid. With the exception of one plasmid from an isolate of L. seeligeri, restriction analysis and hybridization experiments showed a high degree of homology between the different plasmids.

Cloning and heterologous expression of an insecticidal delta-endotoxin gene from Bacillus thuringiensis var. aizawai IC1 toxic to both lepidoptera and diptera

Bacillus thuringiensis var. aizawai IC1 synthesises an insecticidal protein delta-endotoxin (130-135 kDa) that is toxic to both lepidopteran and dipteran larvae, and cross-reacts immunologically with certain monospecific lepidopteran toxins. A 166-kb plasmid from this bacterium was found to hybridise with an intragenic probe derived from the clone B. thuringiensis var. sotto lepidopteran-specific delta-endotoxin gene. A strongly hybridising 5.2-kb SstI fragment from var. aizawai plasmid DNA was cloned in pUC18. After subcloning of this DNA in Escherichia coli, recombinants were obtained that synthesised large amounts of a 130-135-kDa protein. The protein was deposited in the cytoplasm as microscopically visible inclusion bodies and lysates of these cells were found to be toxic to both lepidopteran and dipteran larvae by comparison with controls. The structural basis for the dual specificity of this var. aizawai toxin is now amenable to further study.