In the bacterium Escherichia coli, a group of fve genes code for enzymes required to synthesize the amino acid tryptophan. All fve genes are transcribed together together as a unit called operon.A operon is a group of genes that is under the control of a single operator site. A regulatory protein called a repressor can bind to the operator site and prevent transcription.When the tryptophsn is lacking in the environment, the repressor is made, but it is unable to bind to the !A and block transcription."!A polimerase binds to the promoter site and then proceeds do#n the !A, transcribing the genes for the tryptophan biosynthesis enzymes.When the tryptophan is present in the environment, the organism no longer needs to make tryptophan. $ryptophan binds to the repressor and activates it. $he actived repressor no# bind to the operator, located #ithin the trytophan promoter, and blocks transcription.Attenuationt%verall, attenuation in the trp operon can prevent e&pression of tryptophan biosythesis genes #hen tryptophan level in the cell are high, essentially 'turning o() the trp operon #hen it isn*t needed.Attenuation in the trp operon involves structure and sequences of the trp +rna.Interaction among complementary sequences of the trp, m"!A can form stemloop structure in the "!A trancript. -our region all involved, these are numbered relative tp the .*/end of the transcript, #ith 0 closest to the .*/end."egion 1 is complementary to both region 0 and 2, and can form a stem loop structure #ith either of them, but not #ith both at the same time.3imilarly, region 2 is complementary to both region 1 and 4. When region 0 and 1 form a stem loop, region 2 forms a stem loop #ith region 4. Immediately follo#ing region 4 is a uracil/rich sequence called the attenuator. When the region 2/4 stem loop is present, it acts #ith the attenuator to form a terminator for transcription at the end of trp, gene.When the region 1/2 stem loop is present, the terminator structure does not formand transcription proceeds to the trpE gene and other tryptophan biosynthesis genes in the operon.$here are 2 scenario possible #ith respect to attenuation in the trp operon. If transcription is occuring alone 5in the absence6, then the trp, m"!A #ill take on its most stable conformation.In that form, the region 0/1 and region 2/4 stem loops form and transcription is terminated at the end of trp, gene$he other t#o scenarios involve translation coupled #ith transcription. $he di(erent bet#een these t#o scenario depends on #heather or not the ribosome gets stalled in the region 0 area.$#o ad7ecent tryptophan codons in region 0 determine #heather or not stalling #ill take place there.If tryptophan in the cell are lo#, then trp transfer "!A levels #ill also be lo#. $his#ill couse the ribosome to pause at the t#o tryptophan codons as it #aits for trypto8phan transfer "!A.$he location of the ribosome at this pause #ill shield region 0 from region 1, thusfreeing region 1 to form a stem loop #ith region 2.3ince this prevent the formation of the region 2/4 stem loop, transcription is allo#ed to proceed into the trp biosynthesis genes in the operon.If the tryptophan in the cell are high, then the ribosome #ill not get stalled at thetrp codons in region 0. $he ribosome #ill continue until it reaches the stop codon for the trp, gene, #here it #ill e&perience the normal pause that occurs during translation termination. While pauses the the stop codon, the ribosome #ill physically block region 1 frominteracting #ith region 2$hus the region 2/4 stem loop #ill raridly form, leading to termination of transcription before the trp biosynthesis genes are reched. In this #ay, attenuation turns o( the trp operon #hen trp levels are high.