Expression of acetyl-CoA carboxylase promoter: GUS fusions in developing Arabidopsis thaliana

Abstract

Acetyl-CoA Carboxylase (ACCase) catalyzes the reaction of acetyl-CoA and bicarbonate in order to form malonyl-CoA, which in turn is used as a precursor of many biologically important chemicals (Choi 1995). The significance of this reaction can be demonstrated by the fact that nearly all organisms contain some form of this enzyme, many even contain multiple genes or isoforms. One of the reasons for the widespread presence of ACCase is that the conversion of acetyl-CoA to malonyl-CoA is the first committed step in the production of fatty acids , Wakil 1983. The fatty acids may then be utilized for membranes, structural or protective very long chain fatty acids, or as energy storage.Within bacteria, animal, and fungal cells this is the primary use for malonyl-CoA; plants, however, also use the malonyl-CoA to produce a range of secondary products (Conn 1981. The division of the synthesis of fatty acids and secondary products in plants corresponds to the presence of separate pools of malonyl-CoA within the cytosol and the plastid of the plant cell, whose membranes are impermeable to malonyl-CoA. Since different products are made from the two pools of malonyl-CoA it make sense for the plant to regulate them independently. One way that most plants seem to regulate ACCase and the malonyl-CoA produced is by utilizing isozymes, enzymes encoded by separate genes that differ structurally but have similar functions. Since bacteria, animals, and fungi only have a cytosolic pool of malonyl-CoA they only require a cytosolic ACCase, however, plants are 2 much more complex. Plants contain both a cytosolic and plastidic form of ACCase. The plant's cytosolic isozyme is similar to that found in the animal and fungal kingdoms, and is encoded for by a small gene family. Depending on the plant species, the plastidic isozyme may be similar to the cytosolic form or it may be closer to the ACCase found in bacteria like E. coli (Sasaki 1993, Alban 1994, Konishi 1994). This variability suggests a complex genetic evolution of ACCase within the plant kingdom (Sasaki 1995). In addition to the genetic complexity, the transcription and activation of ACCase allows for more levels ofregulation, which are only beginning to be understood. Acetyl-CoA carboxylase reactionACCase is a member of the biotin containing family of enzymes, a group of enzymes that all require the attachment of a biotin co-factor in order to transfer a carboxyl group between substrates (Knowles 1989). The similarities between ACCase and the other members of this family, such as propionyl-CoA carboxylase, pyruvate carboxylase, 3methylcrotonyl-CoA carboxylase, oxaloacetate decarboxylase, glutaconyl-CoA carboxylase, and transcarboxylase (Caffrey 1995), suggest a common ancestry (Samols 1988), however only some, including ACCase, are widespread in modem organisms. In this group of enzymes biotin is covalently bound to a lysine side chain to form biocytin. From a kinetic standpoint the reaction proceeds in two major steps: first is biotin carboxylation to produce carboxyl-biocytin, ...