The Trp repressor (TrpR) binds to at least five operators in the Escherichia coli genome, repressing gene expression. The operators at which it binds vary considerably in DNA sequence and location within the promoter; when bound to the Trp operon it recognises the sequence 5'-ACTAGT-3' and acts to prevent the initiation of transcription. The TrpR controls the trpEDCBA (trpO) operon and the genes for trpR, aroH, mtr and aroL, which are involved in the biosynthesis and uptake of the amino acid tryptophan . The repressor binds to the operators only in the presence of L-tryptophan, thereby controlling the intracellular level of its effector; the complex also regulates Trp repressor biosynthesis by binding to its own regulatory region. TrpR acts as a dimer that is composed of identical 6-helical subunits, where four of the helices form the core of the protein and intertwine with the corresponding helices from the other subunit.
The bacterial chromosomal replication initiation factor DnaA is a monomeric protein that shows structural similarity to the TrpR, except that it contains additional N-terminal helices. DnaA is a member of the AAA+ family of ATPases, and forms a large, oligomeric assembly at the replication origin site (oriC); the oligomeric complex of DnaA recognises and processes specific origin sequences in order to initiate replication in bacteria .