Gene Regulation And The Lysogenic State Of Phage Lambda
In the research article “Stability and Instability in the Lysogenic State of Phage Lambda” the authors J. W. Little and C. B. Michalowski analyzed the stability of the lysogenic state of phage lambda, including what would cause the phage to, and at what point would, the phage would switch over to a lytic pathway. The article explained that the virus maintains a stable companionship with the host cell, and the lytic actions of the virus are repressed by the viral CI repressor. The CI repressor acts as a transcriptional repressor that allows virus to build and maintain latency.
The bacterial cells harboring a lysogenic lambda phage are unable to be further infected by lambda phages because the CI repressor protein inhibits the lytic development of any additional infecting phages. This state can easily change to the lytic pathway when the host SOS system is induced. The SOS response is a sophisticated mechanism that bacteria evolved to detect and repair DNA damage.
In the SOS response the cell cycle is arrested and DNA repair occurs using the RCA (regulators of complement activation) protein. The RecA protein is involved in the inactivation of LexA which is a transcriptional repressor that represses SOS response genes coding primarily for error-prone DNA polymerase, repair enzymes and cell division inhibitors. In the article the researchers found that “The intrinsic rate of switching to the lytic pathway, measured in a host lacking the SOS response, was abnormally low”. Therefore it was concluded that the low rate did not happen in the process of evolution but resulted from optimizing the rate of switching in a wild-type host over the natural range of SOS-inducing conditions.
Wild type host refers to the phenotype of a species as it occurs in nature. In the article the researchers also analyzed a mutation that was found, named lambda prm240, this promoter controlled the CI expression which was weakened, rendering the lysogens unstable. It was further found that the stability of lambda prm 240 lysogens was based on what growth medium that it was placed into. The lysogens developed in a minimal medium were stable but switched to a higher rate when developed in richer medium. These effects on stability reflected corresponding effects on the strength of the prm240 promoter, measured in an uncoupled assay system. Many derivatives of λprm240 with altered stabilities were characterized. These mutants and its derivatives afford a system for future analysis of stability.
