Concept 33 Genes can be turned on and off.

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Jacques Monod and Francois Jacob work with how bacteria breaks large sugars into smaller pieces.

Bonjour, I’m Jacques Monod. Bonjour, I’m Jacques Monod. And I’m François Jacob. While I was working on my Ph.D. thesis in Paris, I became interested in how bacteria grow. Bacteria use enzymes to break large sugars into smaller pieces. For example, the enzyme b-galactosidase (b-gal) breaks lactose into galactose and glucose. Normally, lactose turns on the gene that produces b-gal. In other words, lactose is an inducer. Lactose regulates the b-gal gene through other intermediates. One of these intermediates is coded by the i gene. The i gene produces an inhibitor that keeps the b-gal gene turned off. Lactose regulates the b-gal gene through other intermediates. One of these intermediates is coded by the i gene. The i gene produces an inhibitor that keeps the b-gal gene off. When lactose is present, it binds to the inhibitor. This prevents the inhibitor from turning off the b-gal gene. Here's how we showed that there is an inhibitor. We first found mutant bacteria that never turn off the b-gal gene. We assumed that these mutants couldn't produce any inhibitor and called them i -. Then we crossed a normal male with the mutant female. Remember, Lederberg’s experiments (Concept 18) showed that males can pass plasmid DNA to females during conjugation. The female mutant also couldn’t produce b-gal; she was b -and i -. The male donated a plasmid with a functioning b-gal (b+) gene to the female. As the b+ gene entered the female, the cell immediately began producing b-gal, because no inhibitor was present. Therefore, wild-type bacteria must produce an inhibitor. Up to this point, we knew that the inhibitor keeps b-gal synthesis turned off until lactose is added. But we didn’t know what the switch was. I reasoned that there must be a binding site for the inhibitor on the DNA itself. We called this binding site the OPERATOR. We were able to find bacteria strains with mutated operators (O-). Though they have working inhibitors (i+), they produce b-gal in the absence of lactose. This is because the operator cannot bind the inhibitor. The model of gene regulation that emerged from all this work, and subsequent work by others, is called the lac operon. When lactose is absent, i produces an inhibitor protein that binds to the operator. This blocks RNA polymerase from binding to the site where mRNA transcription starts. When lactose is present, it binds to the inhibitor and prevents the inhibitor from attaching to the operator. This frees the site for RNA polymerase. mRNA is transcribed and b-gal is made. (Any inhibitor already attached to the operator spontaneously detaches and then gets bound by lactose.) Two other structural proteins important for lactose metabolism are also regulated by this same system. The genes for these proteins lie immediately downstream of the b-gal gene, and are turned on at the same time. The word "operon" refers to this close arrangement of related genes and their common regulation. This part of the lac operon is a classic example of NEGATIVE regulation, because an inhibitor must be removed from the DNA to turn on the gene. The lac operon is also positively regulated. As well as getting rid of the inhibitor, an activator must also attach to the DNA to turn on b-gal synthesis. This only happens when glucose is absent. The lack of glucose stimulates production of a molecule called cAMP. cAMP then attaches to a protein, the cAMP receptor protein (CRP). The cAMP-CRP complex then binds to the DNA and helps RNA polymerase begin transcription. Thus, the lac operon of bacteria demonstrates both positive and negative regulation. Eukaryotes also use these same principles to regulate genes, but the details are more complex.

factoid Did you know ?

In our animation, we called the product of the i gene the inhibitor, but Jacob, Monod, and most textbooks call it the repressor.


How do scientists come up with names for genes?