Advertisement
antibiotic, the plant tissue will show chiorusis, caused by the lack of chlorophyll
synthesis and inhibition of growth (Benveniste and Davies. 1973; Brasileiro. 1998).
2.6.2. Marker genes that confer resistance to herbicides
2.6.2.1. bar a n d p a t genes
The bur and pat genes were isolated from Strrptomyces hygroscopicus and S.
viridochromogenes, respectively. These similar genes code for the enzyme
phosphinothricin-N-acetyltransferase (Murilknmi et al., 1986). PPT, also well know as
ammonium glufosinate, is analogus to glutamate, the substrate of
GS,
and acts as a
competitive inhibitor of GS. The enzyme GS catalyzes the conversion of glutamate to
glutamine, removing the toxic ammonia from the cell. This enzyme plays as essential
role in the regulation of nitrogen metabolism and ammonia assimilation. When the GS
is inhibited, it results in ammonia accumulation and as associated disruption of
chloroplast structure that leads to inhibition of photosynthesis and to death of the
plant cell (Lindsey, 1992).
2.6.2.2. aroA
(or
epsps) gene
This gene was isolated from Salmonella ryphimurium treated with mutagenic
agent and selected for resistance to the herbicide glyphosate (Comai et 01.. 1998). The
herbicide glyphosate inhibits, by competition, the enzyme S-enolpyruvylshikimate-3-
phosphate synthase (EPSPS), which is involved in the enzymatic pathway of aromatic
amino acids biosynthesis in bacteria and plants. The inhibition of EPSPS results in
Advertisement
