Bromoxynil nitrilase

This strategy has led to commercial development of herbicide resistance for glu-fosinate, glyphosate and bromoxynil. Glufosinate and glyphosate resistance will be discussed in detail in later sections of this chapter (see also Chapter 6.2). Bro-moxynil s herbicide activity is due to inhibition of electron transport in photosystem II. Crops engineered with bromoxynil nitrilase metabolize the herbicide to a non-phytotoxic compound [5]. 

Stalker DM, KE McBride (1987) Cloning and expresssion in Escherichia coli of a Klebsiella ozaenae plasmid-borne gene encoding a nitrilase specific for the herbicide Bromoxynil. J Bacterial 169 ... 

Similarly, the selective herbicides, bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and ioxynil (3,5-diiodo-4-hydroxybenzonitrile) are degraded by soil bacteria to their corresponding amide products 3,5-dibromo-4-hydroxybenzamide (BrAM) and 3,5-diiodo-4-hydroxybenzamide (IAM) but are not further degraded to the corresponding acids. The identification of amidases or nitrilases able to effect these transformations, in a soil bacterium, would be of value as a bioremediation agent [48],... 

Transgenic plants containing a nitrilase specific for the herbicide bromoxynil (= 3,5-dibromo-4-hydroxybenzonitrile)have also been developed [93] the Cal-gene company transformed tobacco plants with the bacterial Klebsiella ozaenae gene encoding nitrilase [94] that detoxifies the herbicide by hydrolysis (conversion of bromoxynil to 3,5-dibromo-4-hydroxybenzoic acid), resulting in the establishment of the herbicide-resistant transgenic plants. 

Klebsiella pneumoniae ssp. ozaenae Bromoxynil-specific nitrilase Stalker etal. (1998)... 

Stalker, D.M., Malyi, L.D., and McBride, K.E. 1998. Purification and properties of a nitrilase specific for the herbicide bromoxynil and corresponding nucleotide sequence analysis of the bxn gene. Journal of Biological Chemistry, 263 6310-4. 

A search of sequence libraries made it possible to find a 53 % identity with the nitrilase of Klebsiella pneumoniae active on Bromoxynil (ref. 21). 

Figure 1.6. The conversion of bromoxynil to 3,5-dibromo-4-hydroxybenzoic acid by a nitrilase enzyme.

Another method of increasing resistance to a PSII herbicide is to promote the breakdown of the molecule within the plant before it reaches the active site. This has been achieved in an effective way in the case of bromoxynil resistance in tobacco. Bromoxynil is broken down by a nitrilase enzyme to ammonia and a herbicidally inactive benzoic acid moiety (Figure 1.6). A strain of Klebsiella pneumoniae has been isolated which uses bromoxynil as a primary nitrogen source by virtue of its nitrilase activity. The gene has been characterized and transferred into tobacco using conventional gene transfer techniques. The transgenic tobacco thus produced was able to tolerate more than 20 times the normally lethal dose of bromoxynil. ... 

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