Role of Positive and Negative Feedback in Dynamic Expression of E.coli MarA

Bacteria can stochastically express stress response genes, allowing a subset of a given population to survive sudden changes to the environment. This ‘bet-hedging’ approach maximizes fitness in variable environments without overtaxing the cells. The multiple antibiotic resistance activator (MarA) in Escherichia coli is a transcriptional activator that can trigger more than 40 downstream genes related to stress response. Preliminary data indicates that MarA’s expression is dynamic, using a series of stochastic pulses. We hypothesize that the stochastic pulsing behavior of MarA is driven by the interaction between positive and negative feedback loops controlling the marRAB operon. However, it is not yet clear how the interaction between the two feedback loops affects the pulsing dynamics. Our research goal is to examine how positive and negative feedback regulating marRAB may cause pulsing in MarA expression. To test this, we are constructing systems which contain either only positive or only negative feedback using transversion mutations in the MarR or MarA binding sites in the promoter of the marRAB operon. Single-cell microscopy will be employed to measure the expression of MarA expression levels over time. Previous computational results from our group indicate that pulsing in MarA expression may be caused by the interlinked positive and negative feedback loops. However, with only positive or only negative feedback controlling the marRAB operon, MarA expression will lose the stochastic pulsing behavior, and instead exhibit continuous high or low levels of MarA expression.