Mark Ptashne's A Genetic Switch: Phage Lambda Revisited PDF

By Mark Ptashne

The 1st version of Mark Ptashne's 1986 booklet describing the foundations of gene legislation in phage lambda turned a vintage in either content material and shape, atmosphere a regular of readability and particular prose that has not often been bettered. This version is a reprint of the unique textual content, including a brand new bankruptcy updating the tale to 2004. one of the remarkable new advancements are contemporary findings on long-range interactions among proteins guaranteed to largely separated websites at the phage genome, and an in depth description of the way gene activation works.

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25 which plots, approximately, the activity of PR versus the concentration of repressor. Note that in a lysogen enough repressor is synthesized to repress PR about 1000-fold. Over the first twofold or threefold drop in repressor concentration from this high level, the activity of PR remains unchanged. In effect, repression is buffered against ordinary fluctuations in repressor concentration, so that lysogens are rarely “accidentally” induced. But when the repressor concentration has dropped about fivefold, PR responds dramatically, functioning at about 50% of its fully unrepressed level.

Because information encoding int is destroyed before that encoding xis, this mRNA produces more Xis than Int. This control is called retroregulation. Case 3—Induction In the third case, induction of a lysogen, the phage must make both Int and Xis to ensure its escape from the chromosome following induction. But because we are witnessing the onset of lytic growth we cannot depend on Cll to provide the requisite Int. The solution is as follows: transcription from PL once again produces a mRNA encoding both Int and Xis, but in this case the sib region is not copied onto the end of the mRNA.

W. W. W. Stahl, and R. Weisberg, eds. (New York: Cold Spring Harbor), pp. 123-145. CHAPTER TWO PROTEIN-DNA INTERACTIONS AND GENE CONTROL Regulatory proteins—λ’s repressor and Cro, for example—bind tightly to specific DNA sequences 15-20 base pairs long. Each of these proteins must select its operator site from among the five million or so base pairs of DNA in a bacterium. This chapter examines the structures of these proteins to show how they accomplish this feat. The principle that emerges is simple: the structure of the protein is complementary to the DNA structure—if the DNA sequence is correct the protein and DNA molecules fit together like lock and key.

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