Written by Lori Rowan and Joe Keller
While the lactose and arabinose operons obviously share many similar characteristics, they also differ in several ways. Overall, it appears the process of regulation in the arabinose operon is more complex than that of the lactose operon.
Both of the operons contain promoter regions, but the arabinose operon has two as opposed to the one of the lactose operon. One of the promoters on the arabinose operon, PBAD, is the promoter for the structural genes. The other promoter, PC, is for the regulator genes. When there is glucose available, only a small amount of cAMP is present. As a result, the CRP protein is not able to bind to the CRP binding site in the arabinose operon. In this event, it doesn’t matter if arabinose is present or not. Two AraC proteins bind to both araO2 and araI and then they bind to each other. Due to the significant attraction between the AraC/araI and AraC/araO2, the DNA forms a loop the does not allow the activation of the araBAD promoter by RNAP. Consequently, there is no ara operon expression. The ara expression codes for the different enzymes that are required to catalyze the metabolism of arabinose. The ara genes are basically the equivalent of the lac genes on the lactose operon. But, obviously, they code for different things in order to express their respective products. There is the AraA gene which codes for arabinose isomerase which is necessary in the conversion of arabinose into ribulose. Also, the AraB gene codes for the enzyme ribulokinase that phosphorylates ribulose. Thirdly, the AraD gene codes for ribulose-5-phosphate epimerase. It can then convert ribulose-5-phosphate to xylulose-5-phosphate that can then be metabolized via the pentose phosphate pathway. Conversely, the lactose operon uses the structural genes of lacZ, lacY, and lacA. The lacZ gene codes for beta galactosidase which is essential for the cleavage of lactose. The lacY gene codes for lac permease, which is used to bring lactose into the cell. Finally, transacetylase is encoded by the lacA gene which is required for the transfer of acetyl groups from one compound to another.
Although the majority of characteristics of the lactose and arabinose operons are similar, there are several parts that differ significantly. Clearly, all characteristics cannot be alike considering these operons are ultimately producing different things.