Lakhasly

Genetics of Bt toxins and its Mode of action: In the early 1980s, it was established that most genes coding for the ICPs reside on large transmissible plasmids, of which most are readily exchanged between strains by conjugation [44].Experimental data suggest that the C-terminal and middle domains of the toxin are involved in epithelial cell receptor binding and structural functions, while the N-terminal domain is primarily involved in ion channel and pore formation[56].and can be summarized in the following stages: 1) ingestion of sporulated Bt and ICP by an insect larva; 2) solubilization of the crystalline ICP in the midgut; 3) activation of the ICP by proteases; 4) binding of the activated ICP to specific receptors in the midgut cell membrane; 5) insertion of the toxin in the cell membrane and formation of pores and channels in the gut cell membrane, followed by destruction of the epithelial cells [53].In the midgut of the target larva the parasporal crystalline ICP is dissociated to the protoxin form, and the protoxin is then activated to a biologically active holotoxin by the proteolytic enzymes and specifically the alkaline environment of the gut [55]..Since these initial studies, numerous ICP genes have been cloned, sequenced and used to construct Bt strains with novel insecticidal spectra [45].?-endotoxins are encoded by the Cry and Cyt genes These genes become active during sporulation because they are controlled by a dedicated RNA polymerase that is also synthesized specifically while spores areforming.Pore or ion channel formation occurs after the binding to the receptor and insertion of the N-terminal domain into the membrane, whereby the regulation of the transmembrane electric potential is disturbed.

Genetics of Bt toxins and its Mode of action:
In the early 1980s, it was established that most genes coding for the ICPs reside on large
transmissible plasmids, of which most are readily exchanged between strains by conjugation
[44]. Since these initial studies, numerous ICP genes have been cloned, sequenced and used
to construct Bt strains with novel insecticidal spectra [45]. Δ-endotoxins are encoded by the
Cry and Cyt genes These genes become active during sporulation because they are controlled
by a dedicated RNA polymerase that is also synthesized specifically while spores areforming. Up to 20% of the spore protein content is represented by these Cry ⁄ Cyt toxins [46].
These δ-endotoxins have molecular weights between 14-160 kDa and can be visualized
under light microscopy as
inclusion bodies [47]. In addition, Bt has other insecticidal proteins like Vips that are
secreted during its vegetative cycle [48] Cry genes were classified into 40 families according
to their amino acid sequence similarities [49]. and the sequence of more than 160 cry genes
are known. The specificity of the toxic effect of the δendotoxins against certain species
makes them environmentally friendly tools for the control of insects that are plagues of
important agricultural crops. The currently known crystal (cry) gene types encode ICPs that
are specific to either Lepidoptera (cryI), Diptera and Lepidoptera (cryII), Coleoptera (cryIII),
Diptera (cryIV), or Coleoptera and Lepidoptera (cryV) [50]. A separate designation is used
for the cytolytic (cyt) genes that encode a nonspecific cytolytic factor, present in Bti ICP and
some other Bt subspecies. The sporulated Bt with ICP or spore-ICP complexes must be
ingested by a susceptible insect larva to be effective [51]. The mode of action of Bt has been
reviewed by Schnepf et al. [52].
and can be summarized in the following stages: 1) ingestion of sporulated Bt and ICP by an
insect larva; 2) solubilization of the crystalline ICP in the midgut; 3) activation of the ICP by
proteases; 4) binding of the activated ICP to specific receptors in the midgut cell membrane;
5) insertion of the toxin in the cell membrane and formation of pores and channels in the gut
cell membrane, followed by destruction of the epithelial cells [53]. and 6 subsequent Bt
spore germination and septicaemia may enhance mortality [54].. In the midgut of the target
larva the parasporal crystalline ICP is dissociated to the protoxin form, and the protoxin is
then activated to a biologically active holotoxin by the proteolytic enzymes and specifically
the alkaline environment of the gut [55].. Shortly afterwards, the gut becomes paralysed and
the larva ceases to feed. Experimental data suggest that the C-terminal and middle domains
of the toxin are involved in epithelial cell receptor binding and structural functions, while the
N-terminal domain is primarily involved in ion channel and pore formation[56]. Pore or ion
channel formation occurs after the binding to the receptor and insertion of the N-terminal
domain into the membrane, whereby the regulation of the transmembrane electric potential is
disturbed. This can result in colloid-osmotic lysis of the cells, which is the main cytolytic
mechanism that is common to all ICPs [57].When the midgut epithelium of the larva is
damaged, the haemolymph and gut contents can mix. This results in favourable conditions
for the Bt spores to germinate. The resulting vegetative cells of Bt and the pre-existing
microorganisms in the gut proliferate in the haemocoel causing septicaemia, and may thus
contribute to the mortality of the insect larva