Highlights Transcription II
1. Transcription occurs in three phases – initiation, elongation, and termination.
2. Initiation involves the RNA polymerase binding to the promoter (through the sigma factor) and the synthesis of about the first 10 nucleotides of the RNA.
3. Elongation occurs as the RNA polymerase moves forward, advancing the transcription bubble.
4. Termination of transcription in E. coli occurs by two mechanisms we will be concerned with. One I discussed in class is factor independent transcription termination, which occurs as a result of a hairpin loop forming in the sequence of an RNA. When it forms, it “lifts” the RNA polymerase off the DNA and everything falls apart and transcription stops at that point.
5. Factor dependent termination and is caused by a protein called rho. Rho works by binding to the 5′ end of the RNA and sliding up the RNA faster than the RNA Polymerase makes RNA. When rho catches the RNA polymerase, it causes the RNA polymerase to dissociate (come off of) the DNA and release the RNA.
6. An operon is a collection of genes all under the control of the same promoter. When an operon is transcribed, all of the genes on the operon are on the same mRNA. Operons occur in prokaryotes, but not eukaryotes. In eukaryotes, each gene is made on individual mRNAs and each gene has its own promoter.
7. Operons are prokaryotic arrangements of multiple genes (with common functions) under the control of a single promoter. The lac operon contains genes that E. coli uses for metabolizing the sugar lactose. Control of operons is important. Synthesis of RNA and protein requires considerable energy. Cells can’t afford to waste energy making genes if they don’t need them. Consequently, operons, such as the lac operon must turn on when lactose metabolism is needed and turn off when it isn’t.
8. Lactose metabolism is needed in E. coli when 1) cells are low in energy and 2) when lactose is present. Lactose metabolism is NOT needed in E. coli when 1) cells have plenty of energy and 2) when lactose is not present.
9. Control of the lac operon is accomplished by two proteins that recognize and bind to sequences in the lac promoter – the lac repressor and CAP. The lac repressor binds an inducer molecule made from lactose and CAP binds cAMP. When there is no lactose, the lac repressor cannot bind the inducer, since it is not present, so the lac repressor binds the lac promoter region. Binding of lac repressor to the lac promoter prevents RNA polymerase from binding and stops transcription. When lactose is present, the inducer is made and it binds to the lac repressor and prevents the lac repressor from binding to the lac promoter, allowing RNA polymerase to bind and transcription to start.
10. CAP protein binds a molecule called cAMP. When cAMP is present, it binds to CAP and allows CAP to bind to a region just upstream of the lac promoter. Binding of a molecule causes CAP to bind near the promoter, but binding of a molecule causes the lac repressor to NOT bind near the promoter. Binding of CAP near the promoter HELPS RNA polymerase to bind to the promoter and begin transcription.
11. The tryptophan (called trp) operon of E. coli is interesting for a clever transcriptional termination mechanism known as attenuation. The mRNA of the trp operon can form base pairs in two different ways using four different regions.When tryptophan is abundant, regions 3 and 4 pair together and transcription is quickly terminated before the RNA polymerase gets very far in the operon. Thus, proteins from the rest of the operon cannot be made, since their RNA is not synthesized for translation. On the other hand, if tryphtophan is scarce, transcription termination does NOT occur and the RNA polymerase continues along and copies the entire operon (allowing all the proteins in the operon to be formed by translation of the mRNA).