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Bacteriophage Lambda - Cro and cI

Here is an E. coli cell. It takes that little bugger only 40 hours to colonize the entrails of a newborn baby... but luckily it’s harmless… usually… except for producing gas, which freaking stinks! Now, E. coli are gram negative bacteria, which means they have two membranes with a cell wall in between. Bacteriophage lambda is a nasty little virus that preys upon these unfortunate bacteria. The virus is an obligate parasite, so in order to reproduce, it must inject its DNA into a host cell. First, it must get its genome past the outer membrane. It does this via the protein Maltoporin, which normally recognizes maltose. The fiend injects the DNA, and it passes both through the outer membrane and through the cell wall. Now, to get shuttled through the cytoplasmic membrane, the genome uses the phosphotransferase system. Once the virus’ genome is inside its new, unsuspecting host, it circularizes . At this point, two possible processes can take place… the prophage (or lysogeny) pathway allows the bacteria to live, while the lytic pathway leads to its demise!!! In the prophage pathway, the virus DNA integrates into the host genome and is at this point termed a latent prophage. The host unsuspectingly replicates over and over for however-many generations, taking the virus genome with it. However, in the lytic pathway, the circularized bacteriophage lambda DNA gets busy, using the cell’s replication machinery to make many many copies of itself. Then, one fine day as the E. coli is just swimming around, it senses that something is very wrong. Something is very very w… OOOohhhh! And that, children, is where viruses come from. And then the reproductive process repeats all over again. However, don’t think the bacterium in the prophage pathway got away so easy! That virus is a ticking time bomb. Someday, in one of the E. coli’s babies, and induction event will cause a shift to the lytic pathway. What is an induction event? It is when the host cell responds to DNA damage. At that point, the virus realizes it’s on a sinking ship and begins to replicate while there’s still time. How does the virus either stay in the prophage state or shift to the lytic state? This is thanks to two genes, c1 and Cro. When c1 is active, Cro is repressed, and vise versa. When C1 produces its mRNA, that becomes a protein that then acts on the operator to inhibit Cro and activate the C1 gene. It is important to remember that when c1 is active, most of lambda’s DNA remains untranscribed. When Cro is transcribed, the translate mRNA becomes an evil protein that spell’s the E. coli’s doom. MUAHAHAHA!!! This protein also acts on the operator. It activates the Cro gene, causing yet more protein to be produced, and in this state, most of the virus’s DNA gets transcribed. This protein also inhibits the c1 gene. As a result, the bacterium turns into an evil factory working for the virus until its death, at which point it spreads the virus to other unsuspecting buggers.