Question

b.       What is the polycistron in the Lac operon? In the Trp operon?

d.       Name an aspect of prokaryotic gene regulation that can only occur in a prokaryote and not a eukaryote

e.       Provided the wildtype lac operon, give three different examples of mutations, what they target, and how they disrupt WT regulation of the operon.

f.        Describe how the RNA leader of the Trp operon can repress/induce gene translation

Answer 1

Primate (where RNA polymerases) DNA Bacteria such as Escherichia coli(a friendly inhabitant of our gut) need amino acids to survive-because, like us, they need to build proteins. One of the amino acids they need is tryptophan. If tryptophan is available in the environment, E. coli will take it up and use it to build proteins. However, E. coli can also make their own tryptophan using enzymes that are encoded by five genes. These five genes are located next to each other in what is called the trp operon. If tryptophan is present in the environment, then E. coli bacteria don't need to synthesize it, so transcription of the genes in the trp operon is switched "off." When tryptophan availability is low, on the other hand, the operon is switched "on," the genes are transcribed, biosynthetic enzymes are made, and more tryptophan is produced. Structure of the trp operon The trp operon includes five genes that encode enzymes needed for tryptophan biosynthesis, along with a promoter(RNA polymerase binding site) and an operator (binding site for a repressor protein). The genes of the trp operon are transcribed as a single mRNA. Turning the operon "on" and "off" What does the operator do? This stretch of DNA is recognized by a regulatory protein known as the trp repressor. When the repressor binds to the DNA of the operator, it keeps the operon from being transcribed by physically getting in the way of RNA polymerase, the transcription enzyme. The trp repressor does not always bind to DNA. Instead, it binds and blocks transcription only when tryptophan is present. When tryptophan is around, it attaches to the repressor molecules and changes their shape so they become active. A small molecule like trytophan, which switches a repressor into its active state, is called a corepressor. Ondor Cuide to bude 1 1 I Translation Com calcerium Ertyes (crime wounts) tryptophan boynthesis LOW TRYPTOPHAN: MIOH TRYPTOPHAN A polymerase top, repressor (inactwe) Transcription, DNA Tryptophan No tramiption TPE TD Top Top Top TrpE trp trpctrpB trpa RNA polymerase trp repress When there is little tryptoptian in the cell, on the other hand, the trp repressor is inactive (because no tryptophan is available to bind to and activate it). It does not attach to the DNA or block transcription, and this allows the trp operon to be transcribed by RNA polymerase. In this system, the trp repressor acts as both a sensor and a switch. It senses whether tryptophan is already present at high levels, and if so, it switches the operon to the "off"position, preventing unnecessary biosynthetic enzymes from being made. More trp operon regulation: Attenuation Depending on the class you're taking, or on your own interests, you may also have heard about another form of trp operon regulation called attenuation. Like regulation by the trp repressor, attenuation is a mechanism for reducing expression of the trp operon when levels of tryptophan are high. However, rather than blocking initiation of transcription, attenuation prevents completion of transcription. When levels of tryptophan are high, attenuation causes RNA polymerase to stop prematurely when it's transcribing the trp operon. Only a short, stubby mRNA is made, one that does not encode any tryptophan biosynthesis enzymes. Attenuation works through a mechanism that depends on coupling