Tryptophan Operon

Free online life science study material. Tryptophan Operon It is a repressible operon. Tryptophan synthesising genes are off when tryptophan is present in the medium. When trp is high it binds to aporepressor and binds to operator and hence blocks transcription(RNAP binding is blocked). Trp is corepressor. When trp is low in concentration, the operon should be activated for trp synthesis, hence the aporepressor binds to another aporepressor molecule (dimerise) and its confirmation gets changed due to which it can not bind to Operator of operon. Therefore, operon gets activated. Trp operon is weak operon, fully repressed level is not achieved with this repression hence, another control is required which is called attenuation. Attenuation is rho dependent. There is a trp R gene, then trp Promoter and Operator are present, trp O is overlapping trp P. Then five genes for trp synthesis are present trpE,trpD,trpC, trpB and trpA. Two loci, trp leader sequence and trp attenuator lie between Operator and trpE. Attenuator contains transcription stop signal, an inverted repeat followed by string of A's. In leader sequence two trp codons are present.There are four elements present 1,2,3 and 4. During trp starvation, 2 pairs with 3 and the transcription for trp continues. Hairpin termination does not form and it is called antitermination. when trp is present termination is required and is achieved by pairing of 1 with 2 and 3 with 4.3-4 pairing forms antitermination loop with G-C rich region followed by string of U. Due to pairing of 1 and 2 the ribosome falls off and transcription is halted. (Consult diagram for better understanding).