Agriculture Reference
In-Depth Information
Accumulated OAS dissociates from the CSC complex, which rapidly decreases
SAT activity (Hell and Wirtz 2008 ). High OAS concentration increases the expres-
sion of sulfate transporter genes, APR, SAT and OAS-TL. This leads to increased
sulfate uptake and reduction and equilibrates the system (Hopkins et al. 2005 ;
Koprivova et al. 2000 ; Smith et al. 1997 ). Hubberten et al. ( 2012 ) have shown
recently that OAS may serve as a signalling molecule and change the transcription
levels of specific genes irrespective of the sulfur status in the plant. The feedback
inhibition of the cytosolic isoform of SAT by cysteine content serves as another
form of regulation in Arabidopsis . It is important to note that in A. thaliana ,
plastidial and mitochondrial isoforms of SAT remain mostly insensitive to the
changes of cysteine content (Noji et al. 1998 ). Cytosolic SAT activity may be
considered as important for control of OAS concentration. The cysteine insensitive
SAT isoforms in organelles may allow independent formation of cysteine (Noji
et al. 1998 ). Although the regulation of cysteine biosynthesis is very important for
sulfur homeostasis in plants and therefore it has been intensively studied over the
past few years, many important aspects still require further investigation.
Control of Methionine Biosynthesis - Post-transcriptional
Regulation
Methionine biosynthesis is subject to complex regulation. The whole pathway is
controlled by feedback inhibition of aspartate kinase by lysine, threonine or lysine
together with SAM. It seems that the competition between CGS and TS for their
common substrate, O -phosphohomoserine, has an important regulatory role in the
flux of carbon into methionine, which can be a limiting factor in the process (Hesse
et al. 2004a ). A recent study of transgenic Arabidopsis plants provided evidence
that the regulation of methionine biosynthesis also occurs at the posttranscriptional
level (Chiba et al. 1999 , 2003 ). The analysis of this process is focused on the MTO1
region in exon 1 of CGS (Suzuki et al. 2001 ). The MTO1 mRNA region may act in
cis and destabilise CGS mRNA in response to high concentrations of methionine or
SAM (Chiba et al. 1999 , 2003 ; Lambein et al. 2003 ). Computational analysis
revealed the possible formation of a stable stem-loop structure in the MTO1 region
which could support the posttranscriptional mechanism of regulation (Amir
et al. 2002 ). More recently the presence of a truncated form of CGS transcript in
Arabidopsis was demonstrated (Hacham et al. 2006 ). This transcript lacks about
90 nucleotides from the first exon. Over-expression of this CGS transcript causes
even higher levels of methionine than the over-expression of full length CGS. This
may suggest that this truncated transcript is not subject to feedback regulation by
methionine (Hacham et al. 2006 ).
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