Fatty Acid and Lipid Biosynthetic Genes Are Expressed at Constant Molar Ratios But Different Absolute Levels during Embryogenesis

Abstract

In plants, fatty acid and complex lipid synthesis requires the correct spatial and temporal activity of many gene products. Quantitative northern analysis showed that mRNA for the biotin carboxylase subunit of heteromeric acetyl-coenzyme A carboxylase, fatty acid synthase components (3-oxoacyl-acyl carrier protein [ACP] reductase, enoyl-ACP reductase, and acyl-ACP thioesterase), and stearoyl-ACP desaturase accumulate in a coordinate manner during Brassica napus embryogenesis. The mRNAs were present in a constant molar stoichiometric ratio. Transcript abundance of mRNAs for the catalytic proteins was found to be similar, whereas the number of ACP transcripts was approximately 7-fold higher. The peak of mRNA accumulation of all products was between 20 and 29 d after flowering; by 42 d after flowering, the steady-state levels of all transcripts fell to about 5% of their peak levels, which suggests that the mRNAs have similar stability and kinetics of synthesis. Biotin carboxylase was found to accumulate to a maximum of 59 fmol mg Ϫ1 total RNA in embryos, which is in general agreement with the value of 170 fmol mg Fatty acids are synthesized by a common biochemical pathway in all organisms. In plants, de novo synthesis takes place in plastids, using two enzyme systems: acetyl-CoA carboxylase (ACCase) and fatty acid synthase (FAS). The type II FAS, of plants, is composed of separate soluble enzymes that each carry out a single enzymatic reaction (Caughey and Kekwick, 1982; Hoj and Mikkelsen, 1982;Shimakata and Stumpf, 1982) with the growing acyl chains attached to acyl carrier protein (ACP). This is in contrast to the arrangement in animals and yeast where the enzyme activities and ACP are located on one or two multifunctional polypeptides (type I FAS).Acetyl-CoA is carboxylated to malonyl-CoA in plastids of non-graminaceous plants by a heteromeric ACCase, which is encoded by four subunits (Sasaki et al., 1993(Sasaki et al., , 1995. The ␣-carboxyltransferase (␣-CT) polypeptide is encoded in the plastid genome (Sasaki et al., 1993), whereas the ␤-carboxyltransferase (␤-CT), biotin carboxyl carrier protein (BCCP) and biotin carboxylase (BC) subunits are nuclear encoded. MalonylCoA undergoes a thiol-exchange reaction carried out by malonyl-CoA:ACP transacylase (MCAT) to form malonyl-ACP, before condensation with acetyl-CoA. The initial condensation reaction, catalyzed by ketoacyl-ACP synthase (KAS) III, is unique in that malonyl-ACP reacts with acetyl-CoA. Subsequent condensations add two carbon units from malonyl-ACP to the saturated acyl-ACP chain by the action of KAS isoforms I and II. The 3-oxo group is sequentially reduced to a methylene group by the action of 3-oxoacyl-ACP reductase (KR), hydroxyacyl-ACP dehydrase (DH), and enoyl-ACP reductase (ENR) via hydroxyl and enoyl intermediates, before a further condensation reaction takes place.When the acyl chain is 16 or 18 carbons long, several possible reactions occur in plastids. The saturated acyl-ACP may have a double bond introduced, between carbons 9 and 10, by...

LEAFY COTYLEDON1Is a Key Regulator of Fatty Acid Biosynthesis in Arabidopsis

“…However, expression of the plastidial genome-encoded b-subunit remained nearly unaltered. This result is not fully in agreement with previous observations that the expression of all four subunit genes of ACCase is closely coordinated to maintain a constant molar stoichiometric ratio of the transcript concentrations (Ke et al, 2000;O'Hara et al, 2002;Sasaki and Nagano, 2004). We speculate that a functionally excessive amount of b-subunit transcript or protein is present in plastids, thereby sufficing for an increased ACCase activity caused by the elevated levels of the other three subunits.…”

Multifunctional acetyl‐CoA carboxylase 1 is essential for very long chain fatty acid elongation and embryo development in Arabidopsis

“…This is consistent with previous RT-PCR (Yanai et al, 1995) and in situ hybridisation (Nikolau and Wurtele, 1998) analyses. During seed development, the ACC1 expression level appears stable, in contrast with some other genes involved in triglyceride biosynthesis, such as fatty acid synthase subunits, that have been shown to increase during the lipid deposition phase (O'Hara et al, 2002). This raises the question of a possible post-translational regulation of the multifunctional ACCase.…”

Arabidopsis Genes Involved in Acyl Lipid Metabolism. A 2003 Census of the Candidates, a Study of the Distribution of Expressed Sequence Tags in Organs, and a Web-Based Database

“…only genes with a probability greater than 0.99, etc.). (O'Hara et al, 2002), especially when dealing with small EST numbers. Finally, it should be stressed that the probability of the statistical method used here is influenced not only by the EST number but also by the size of the library (for example, see Table II At2g19450 and At3g18280, two genes with the same number of ESTs but a different probability).…”