Department of Biochemistry, Iowa State University, Ames, Iowa 50011, USA.
Plant Physiol. 2011 Jan;155(1):293-314. doi: 10.1104/pp.110.165910. Epub 2010 Oct 28.
The heteromeric acetyl-coenzyme A carboxylase catalyzes the first and committed reaction of de novo fatty acid biosynthesis in plastids. This enzyme is composed of four subunits: biotin carboxyl-carrier protein (BCCP), biotin carboxylase, α-carboxyltransferase, and β-carboxyltransferase. With the exception of BCCP, single-copy genes encode these subunits in Arabidopsis (Arabidopsis thaliana). Reverse-genetic approaches were used to individually investigate the physiological significance of the two paralogous BCCP-coding genes, CAC1A (At5g16390, codes for BCCP1) and CAC1B (At5g15530, codes for BCCP2). Transfer DNA insertional alleles that completely eliminate the accumulation of BCCP2 have no perceptible effect on plant growth, development, and fatty acid accumulation. In contrast, transfer DNA insertional null allele of the CAC1A gene is embryo lethal and deleteriously affects pollen development and germination. During seed development the effect of the cac1a null allele first becomes apparent at 3-d after flowering, when the synchronous development of the endosperm and embryo is disrupted. Characterization of CAC1A antisense plants showed that reducing BCCP1 accumulation to 35% of wild-type levels, decreases fatty acid accumulation and severely affects normal vegetative plant growth. Detailed expression analysis by a suite of approaches including in situ RNA hybridization, promoter:reporter transgene expression, and quantitative western blotting reveal that the expression of CAC1B is limited to a subset of the CAC1A-expressing tissues, and CAC1B expression levels are only about one-fifth of CAC1A expression levels. Therefore, a likely explanation for the observed unidirectional redundancy between these two paralogous genes is that whereas the BCCP1 protein can compensate for the lack of BCCP2, the absence of BCCP1 cannot be tolerated as BCCP2 levels are not sufficient to support heteromeric acetyl-coenzyme A carboxylase activity at a level that is required for normal growth and development.
异源二聚体乙酰辅酶 A 羧化酶催化质体中新脂肪酸生物合成的第一步和关键反应。该酶由四个亚基组成：生物素羧基载体蛋白（BCCP）、生物素羧化酶、α-羧基转移酶和β-羧基转移酶。除 BCCP 外，拟南芥（Arabidopsis thaliana）的这些亚基均由单拷贝基因编码。采用反向遗传学方法分别研究了两个同源 BCCP 编码基因 CAC1A（At5g16390，编码 BCCP1）和 CAC1B（At5g15530，编码 BCCP2）的生理意义。完全消除 BCCP2 积累的转座子插入等位基因对植物生长、发育和脂肪酸积累没有明显影响。相比之下，CAC1A 基因的转座子插入无效等位基因是胚胎致死的，并对花粉发育和萌发有不利影响。在种子发育过程中，cac1a 无效等位基因的影响在开花后 3 天首先显现，此时胚乳和胚胎的同步发育被打乱。对 CAC1A 反义植物的表征表明，将 BCCP1 的积累减少到野生型水平的 35%，会降低脂肪酸的积累并严重影响正常的植物生长。通过一系列方法包括原位 RNA 杂交、启动子：报告基因转基因表达和定量 Western blot 进行的详细表达分析表明，CAC1B 的表达仅限于 CAC1A 表达组织的一部分，并且 CAC1B 的表达水平仅约为 CAC1A 表达水平的五分之一。因此，这两个同源基因之间观察到的单向冗余的一个可能解释是，虽然 BCCP1 蛋白可以补偿 BCCP2 的缺乏，但不能容忍 BCCP1 的缺乏，因为 BCCP2 的水平不足以支持异源二聚体乙酰辅酶 A 羧化酶活性达到正常生长和发育所需的水平。
