The purine repressor (PurR) is a DNA-binding protein involved in the process of transcription. Its name indicates its function: it represses the synthesis of purines. Characterization of PurR has been accomplished primarily by studying its function in bacteria, mainly Escherichia coli. Little is presently known about purine gene expression in humans and other eukaryotes. [top]
Purine nucleotides are important in cellular metabolism. Cells require them to serve as the monomeric substrates for the synthesis of polynucleotides and enzymatic cofactors, to carry the high-energy phosphate bonds necessary to catalyze enzymatic reactions, and to function as second messengers within a cell. Purine nucleotides play an important role in many biochemical processes. ATP is the cell's main energy source. Adenine nucleotides serve as the components of coenzymes: Coenzyme A, FAD and NAD+. GTP is needed for activation of biochemical processes. In addition, the nucleotides serve as the building blocks of DNA and RNA. [top]
Cells synthesize purine nucleotides in two ways. The de novo synthesis proceeds via a complex pathway after production of IMP. The other way is utilization of ready made purine bases in the salvage pathway. In these reactions, ribose phosphate is coupled to purine bases to produce IMP, AMP or GMP. Except for viruses and certain parasites, all organisms can synthesize purine nucleotides de novo. The pathway is invariable, but the organization and regulation of the participating genes differ among organisms. [top]
How is purine synthesis regulated by PurR?
The mechanism for regulation of purine synthesis is that gene expression is repressed by PurR when cells are grown in medium with purines. When sufficient purines are present, the purine bases hypoxanthine or guanine (corepressors) interact with PurR to promote DNA binding. When the repressor is bound, RNA polymerase is unable to recognize promoter sequences and transcription is prevented. [top]
PurR DNA binding...
