Welcome to the ScienceDuo blog by Chris Wallis and Rhiannon Morris. Screeds on science and sanity from two people who understand neither.
Continuing our theme of science SNPs I wanted to talk about a classic model of gene regulation in λ Phage which was elucidated by some very elegant experiments that have formed the bedrock of our understanding of all gene regulation in viruses, bacteria and eukaryotes. It is quite under-rated but A Genetic Switch by Mark Ptashne is one of my favourite molecular genetics books, a must read for anyone interested in gene regulation. It details the molecular mechanisms of the CI/Cro switch that dictates the life cycle of the Lambda Phage.
Firstly, it’s important to note that the lambda phage has two kinds of life cycle, lytic and lysogenic (Figure 1.) Lysogenic growth involves inserting the viral genome into the bacterial chromosome after initial infection via homologous recombination, once integrated the viral genes are silenced and the viral genome is replicated along with the hosts. The virus is dormant and causes no damage to the host cell. Lytic growth on the other hand, involves the switching on and expression of viral genes, which destroys the host chromosome and hijacks the host’s transcriptional and translational machinery to make many copies of new Phage which then burst from the cell to spread to new ones.
So how does the Phage switch between lytic and lysogenic modes? A simple and elegant switch containing two diverging genes that share a regulatory (operator) region is all that is required. The first gene, CI encodes a repressor protein with a duel function which binds to various operator sites throughout the phage genome including its own promoter. The second gene encodes Cro, a repressor protein that inhibits transcription of CI.
Under lysogenic conditions the CI repressor binds three operator sites (Figure 1.) O1, O2 and O3 with different affinities. CI binds O1 very strongly, then O2 with slightly less affinity and O3 quite weakly. The effect of this is that O1 and O2 are occupied most of the time, this prevents transcription of Cro and promotes transcription of more CI (O3 is only occupied under very high CI concentrations.) This configuration ensures that a constant supply of CI is available to bind and silence all other Phage genes including Cro until the lytic switch is triggered.
The switch is flipped and lytic growth is activated by an environmental stimuli such as U.V which cause CI to be degraded. Once the concentration of CI is below a certain threshold and operator sites O1 and O2 are unoccupied, RNA polymerase begins transcription of the Cro and other genes. The Cro protein also binds O1, O2 and O3 with different affinities, except its binding affinity is opposite to CI (O1< O2 < O3.) The effect of this is that O2 and O3 are occupied, which blocks transcription of CI.
After important genes have been turned on, the viral genome is excised from the host, new phage are manufactured and the host cell is ruptured to release a new crop of phage, ready to repeat the whole cycle again.
Figure 1. Left- lytic and lysogenic growth cycles for the Lambda Phage (Campbell, A. (2003). The future of bacteriophage biology. Nature Reviews Genetics, 4(6), 471-477.) Right- structure of the CI/Cro genetic switch (Ptashne, M. (2011). Principles of a switch. Nature chemical biology, 7(8), 484-487.)