Imagine that you are studying the promoters of genes in E. coli. You notice that genes that are expressed in different conditions have different -10 and -35 sequences in their promoters. The genes expressed in condition B have -35 sequences of TTGACA, and -10 sequences of TATAAT. The genes expressed in condition C have -35 sequences of CTGGTA, and -10 sequences of TTGCA. What regulatory system is allowing these genes to be expressed in different conditions? The Catabolic Activator Protein, CAP. When CAMP is present, CAP binds the -10 and -35 sequences in the promoters of the genes expressed in condition B, and when CAMP is not present, CAP binds and activates transcription of the genes expressed in condition C. LysR regulators - a LysR regulator binds and activates transcription of certain genes in condition B, and, when a co-inducer is present, in activates transcription of other genes in condition C. O Lacl binds to the -10 and -35 sequences, and it has a different binding affinity for the different sequences, so it will repress the B genes more than it represses the C genes. Sigma factors - the sigma factor for condition B helps initiate the transcription of the genes expressed in condition B, and the sigma factor for condition C helps initiate the transcription of the genes expressed in condition C.

RNA knockdown has become a powerful tool in the arsenal of methods used to repress gene expression. Briefly describe how gene expression can be knocked down. What effect would introducing siRNAs to TSC1 have on human cells?

CD3 is a signaling protein that is typically found only in the plasma membrane of immune system T lymphocytes. CD3 is composed of several different polypeptides, including a gamma chain, CD3γ. Scientists analyzed the promoter of the CD3γ chain gene for regulatory sequences that might have positive or negative effects on expression of the gene. The scientists cloned fragments of the CD3γ gene that included the first transcribed nucleotides plus up to 789 nucleotides of upstream regulatory sequences into plasmids in which the gene for the firefly enzyme luciferase immediately follows the fragments. The plasmids were then introduced into a line of T lymphocytes (Figure 1), and the cells were allowed to grow for a short while. Because the regulatory sequences of the CD3γ gene immediately precede the luciferase gene in the plasmids, the activity, either positive or negative, of the regulatory sequences affected the amount of luciferase gene expression by the T lymphocytes. Luciferase…