Cry4aa and Cry4ba From Bacillus Thuringiensis Subsp. Israelensis Expressed in Insect Cells by Recombinant Baculoviruses Are Toxic to Aedes Aegypti Larvae

Abstract

Bacillus thuringiensis subsp. israelensis (Bti) is highly toxic to mosquito larvae. Most of its toxicity relies on δ-endotoxins (Cry and Cyt) crystalline inclusions produced at the sporulation phase of growth. Obtention of individual Bti mosquitocidal toxins is crucial for the understanding of Bti´s potential activity against dipteran larvae. A Cry toxin expression system based on baculovirus was developed to produce two of the Cry toxins comprised in the crystals of Bti. Th e cry4Aa and cry4Ba genes from two Brazilian strains of Bacillus thuringiensis were amplifi ed by PCR, cloned into a plasmid cloning vector and sequenced. Sequence analysis of the cry4Aa and cry4Ba genes showed high identity to previous known cry genes. Both cry4Aa and cry4Ba genes were further cloned into transfer vectors for construction of recombinant baculoviruses.. Aft er isolation of the recombinant viruses, they were used to infect insect cells (BTI-TN5B1-4) that were analyzed by light microscopy at diff erent times post infection. Putative crystals consisting of either Cry4Aa or Cry4Ba were observed in the cytoplasm of infected insect cells. RT-PCR was performed with mRNA from insect cell extracts (72 h.p.i.) in order to confi rm the presence of the genes specifi c transcripts. Recombinant virus-infected insect extracts (120 h.p.i.) were analyzed by polyacrylamide gel electrophoresis (SDS-PAGE) showing the presence of polypeptide bands of around 128 and 130 kDa, corresponding, respectively to the sizes of the proteins Cry4Aa and Cry4Ba. Bioassays with virus-infected insect extracts were shown to be toxic to second instar Aedes aegypti larvae, confi rming the usefulness of this expression system for the study of Cry proteins.