Bacillus thuringiensis (Bt) is a Gram positive, spore-forming bacterium that synthesizes parasporal crystalline inclusions containing Cry and Cyt proteins, some of which are toxic against a wide range of insect orders, nematodes and human-cancer cells. These toxins have been successfully used as bioinsecticides against caterpillars, beetles, and flies, including mosquitoes and blackflies. Bt also synthesizes insecticidal proteins during the vegetative growth phase, which are subsequently secreted into the growth medium. These proteins are commonly known as vegetative insecticidal proteins (Vips) and hold insecticidal activity against lepidopteran, coleopteran and some homopteran pests. A less well characterized secretory protein with no amino acid similarity to Vip proteins has shown insecticidal activity against coleopteran pests and is termed Sip (secreted insecticidal protein). Bin-like and ETX_MTX2-family proteins (Pfam PF03318), which share amino acid similarities with mosquitocidal binary (Bin) and Mtx2 toxins, respectively, from Lysinibacillus sphaericus, are also produced by some Bt strains. In addition, vast numbers of Bt isolates naturally present in the soil and the phylloplane also synthesize crystal proteins whose biological activity is still unknown. In this review, we provide an updated overview of the known active Bt toxins to date and discuss their activities.

The 154-kb plasmid was cured from race 7 strain 1449B of the phytopathogenPseudomonas syringaepv. phaseolicola (Pph). Cured strains lost virulence toward bean, causing the hypersensitive reaction in previously susceptible cultivars. Restoration of virulence was achieved by complementation with cosmid clones spanning a 30-kb region of the plasmid that contained previously identified avirulence (avr) genesavrD,avrPphC, andavrPphF. Single transposon insertions at multiple sites (including one located inavrPphF) abolished restoration of virulence by genomic clones. Sequencing 11 kb of the complementing region identified three potential virulence (vir) genes that were predicted to encode hydrophilic proteins and shared thehrp-box promoter motif indicating regulation by HrpL. One gene achieved partial restoration of virulence when cloned on its own and therefore was designatedvirPphAas the first (A) gene fromPphto be identified for virulence function. In soybean,virPphAacted as anavrgene controlling expression of a rapid cultivar-specific hypersensitive reaction. Sequencing also revealed the presence of homologs of the insertion sequence IS100fromYersiniaand transposase Tn501fromP. aeruginosa. The proximity of severalavrandvirgenes together with mobile elements, as well as G+C content significantly lower than that expected forP. syringae, indicates that we have located a plasmid-borne pathogenicity island equivalent to those found in mammalian pathogens.

The avrPphF gene was cloned from Pseudomonas syringae pathovar phaseolicola ( Pph ) races 5 and 7, based on its ability to confer avirulence towards bean cultivars carrying the R1 gene for halo‐blight resistance, such as Red Mexican. avrPphF comprised two open reading frames, which were both required for function, and was located on a 154 kb plasmid (pAV511) in Pph . Strain RW60 of Pph , lacking pAV511, displayed a loss in virulence to a range of previously susceptible cultivars such as Tendergreen and Canadian Wonder. In Tendergreen virulence was restored to RW60 by avrPphF alone, whereas subcloned avrPphF in the absence of pAV511 greatly accelerated the hypersensitive resistance reaction caused by RW60 in Canadian Wonder. A second gene from pAV511, avrPphC , which controls avirulence to soybean, was found to block the activity of avrPphF in Canadian Wonder, but not in Red Mexican. avrPphF also conferred virulence in soybean. The multiple functions of avrPphF illustrate how effector proteins from plant pathogens have evolved to be recognized by R gene products and, therefore, be classified as encoded by avirulence genes.

Bacterial pathogens exhibit significant variation in their genomic content of virulence factors. This reflects the abundance of strategies pathogens evolved to infect host organisms by suppressing ...