explain about cry gene
Answers
Answer:
The cry gene family is a large, still-growing family of homologous genes, in which each gene encodes a protein with strong specific activity against only one or a few insect species. ... Receptor binding is the major determinant of host specificity by different Cry proteins.
Answer:
The cry gene family, produced during the late exponential phase of growth in Bacillus thuringiensis, is a large, still-growing family of homologous genes, in which each gene encodes a protein with strong specific activity against only one or a few insect species.
Explanation:
Introduction
Bacillus thuringiensis, a naturally occurring Gram-positive bacterium, can produce parasporal crystal inclusions consisting of one or several δ-endotoxins (Cry proteins), which have insecticidal properties affecting a selective range of insect orders (1). The cry gene family is a large, still-growing family of homologous genes, in which each gene encodes a protein with strong specific activity against only one or a few insect species. When the crystal proteins are ingested by insects, they are solubilized in the alkaline environment of the insect gut, releasing their constituent Cry proteins as protoxins. Midgut subsequently converts the protoxins into biologically active toxins by proteolytic enzymes. These activated toxins then bind to specific receptors on the surface of the midgut epithelial cells and insert into the cell membranes, forming pores and channels in the gut cell membrane, followed by destruction of the epithelial cells 2., 3.. Receptor binding is the major determinant of host specificity by different Cry proteins. In view of such properties, B. thuringiensis is developed as a type of microbial insecticide and has already been a useful alternative or supplement to synthetic chemical pesticide application on commercial agriculture, forest management, and mosquito control. It is also a key source of genes for pest-resistant transgenic plants (1).
To date, more than 150 different Cry toxins have been cloned and tested, and a new nomenclature has been formulated to accommodate the growing list of new toxin genes/proteins according to their amino acid sequence identities (4). Five tertiary structures of Cry proteins have been determined through X-ray crystallography, namely Cry1Aa, Cry2Aa, Cry3Aa, Cry3Bb, and Cry4Ba. They all suggest that the active toxins are globular molecules with three conserved domains. Domain I, comprising seven α-helices, is thought to be responsible for inserting into insect cell membrane and to be involved in pore formation 5., 6.. Domain II consists of three anti-parallel β-sheets, each terminating in surface-exposed loops, which are the most variable part in the Cry toxin structure. This domain has been demonstrated to participate in receptor recognition and hence determines the insect specificity 1., 7.. Domain III is made of two anti-parallel β-sheets into β-sandwich structure. The role of Domain III at the molecular level is still unknown, although a variety of mutagenesis experiments have shown that it can also be involved in receptor binding and specificity determination 8., 9..
In comparison to the extensive studies describing the function and the specificity determination regions, comprehensive evolutionary analyses of the cry gene family are rather rare. Positive selection is a phenomenon favoring the retention of mutations that are beneficial to an individual or population, and is thought to be an ephemeral case frequently resulting in the occurrence of a protein with novel function (10). The nonsynonymous to synonymous substitution rate ratio (ω = dN/dS) provides a sensitive and effective measure of selective pressure at the protein level, with ω values < 1, = 1, and > 1 indicating purifying (negative) selection, neutral evolution, and positive selection, respectively. The specific goal of this study is to identify whether the cry gene family has been subjected to positive selection in the process of evolution by using the maximum likelihood models of codon substitution implemented in the PAML package (11). The major advantage of these models is that they can account for variable selective pressures by assuming a statistical distribution of the ω ratio among sites (12). Here, the likelihood ratio test (LRT) is utilized for some clades of Cry proteins to study the evolution of the cry gene family and elucidate potential factors that drive the diversification of Cry proteins in B. thuringiensis. We believe that identification of the selection pressures acting on cry genes together with functional and structural data will shed light on their evolution processes and functional implications.