L-arabinose operon

The L-arabinose operon, also called the ara or araBAD operon, is an operon required for the breakdown of the five-carbon sugar, L-arabinose, in Escherichia coli. The L-arabinose operon contains three structural genes: araB, araA, araD (collectively known as araBAD), which encode for three metabolic enzymes that are required for the metabolism of L-arabinose. AraB (ribulokinase), AraA (an isomerase), AraD (an epimerase) produced by these genes catalyse conversion of L-arabinose to an intermediate of the pentose phosphate pathway, D-xylulose-5-phosphate. The L-arabinose operon, also called the ara or araBAD operon, is an operon required for the breakdown of the five-carbon sugar, L-arabinose, in Escherichia coli. The L-arabinose operon contains three structural genes: araB, araA, araD (collectively known as araBAD), which encode for three metabolic enzymes that are required for the metabolism of L-arabinose. AraB (ribulokinase), AraA (an isomerase), AraD (an epimerase) produced by these genes catalyse conversion of L-arabinose to an intermediate of the pentose phosphate pathway, D-xylulose-5-phosphate. The structural genes of the L-arabinose operon are transcribed from a common promoter into a single transcript, a mRNA. The expression of the L-arabinose operon is controlled as a single unit by the product of regulatory gene araC and the catabolite activator protein (CAP)-cAMP complex. The regulator protein AraC is sensitive to the level of arabinose and plays a dual role as both an activator in the presence of arabinose and a repressor in the absence of arabinose to regulate the expression of araBAD. AraC protein not only controls the expression of araBAD but also auto-regulates its own expression at high AraC levels. L-arabinose operon is composed of structural genes and regulatory regions including the operator region (araO1, araO2) and the initiator region (araI1, araI2). The structural genes, araB, araA and araD, encode enzymes for L-arabinose catabolism. There is also a CAP binding site where CAP-cAMP complex binds to and facilitates catabolite repression, and results in positive regulation of araBAD when the cell is starved of glucose. The regulatory gene, araC, is located upstream of the L-arabinose operon and encodes the arabinose-responsive regulatory protein regulatory protein AraC. Both araC and araBAD have a discrete promoter where RNA polymerase binds and initiates transcription. araBAD and araC are transcribed in opposite directions from the araBAD promoter (PBAD) and araC promoter (PC) respectively. Both L-ribulose 5-phosphate and D-xylulose-5-phosphate are metabolites of the pentose phosphate pathway, which links the metabolism of 5-carbon sugars to that of 6-carbon sugars. The L-arabinose system is not only under the control of CAP-cAMP activator, but also positively or negatively regulated through binding of AraC protein. AraC functions as a homodimer, which can control transcription of araBAD through interaction with the operator and the initiator region on L-arabinose operon. Each AraC monomer is composed of two domains including a DNA binding domain and a dimerisation domain. The dimerisation domain is responsible for arabinose-binding.AraC undergoes conformational change upon arabinose-binding, in which, it has two distinct conformations. The conformation is purely determined by the binding of allosteric inducer arabinose. AraC can also negatively autoregulate its own expression when the concentration of AraC becomes too high. AraC synthesis is repressed through binding of dimeric AraC to the operator region (araO1). When arabinose is absent, cells do not need the araBAD products for breaking down arabinose. Therefore, dimeric AraC acts as a repressor: one monomer binds to the operator of the araBAD gene (araO2), another monomer binds to a distant DNA half site known as araI1. This leads to the formation of a DNA loop. This orientation blocks RNA polymerase from binding to the araBAD promoter. Therefore, transcription of structural gene araBAD is inhibited. Expression of the araBAD operon is activated in the absence of glucose and in the presence of arabinose. When arabinose is present, both AraC and CAP work together and function as activators.