| Numerous bacteria have evolved the ability to condition culture medium with growth-regulatory signaling compounds for coordinated expression of specific genes with cell cycle, a phenomenon called autoinduction or quorum sensing. Recently, the autoinducer-2 (AI-2)-potentiated signaling system of Vibrio harveyi was used to demonstrate that several bacterial species, including Escherichia coli, produce an extracellular factor identical to AI-2 of V. harveyi. Using glass DNA microarrays, we identified two-hundred and forty-two genes (∼5.6% of the E. coli genome) which exhibited significant transcriptional changes (induction or repression) in response to AI-2 signaling with fifty-one of the genes displaying greater than 5-fold changes in expression. Additionally, our findings suggest that AI-2 and a transcriptional activator of cell division, SdiA, comprise a multi-component system able to sense intracellular and environmental stimuli and subsequently channel this information into the quorum circuit for physiological and/or morphological adaptation to stress and starvation. It was further demonstrated that AI-2 activity increased with increasing culture growth rate and that perturbations that typically inhibit growth rate, such as chemical stressors (i.e. ethanol, acetate) and abnormal protein production, resulted in significant degradation or inhibition of AI-2 signaling. We showed that the dynamics of AI-2 signaling were linearly related to the induction level of eight protein products and that increased rate of recombinant protein production resulted in decreased level of AI-2 signaling. Importantly, our findings suggest that recombinant E. coli communicate the stress or burden of overexpressing heterologous genes through the AI-2 signaling pathway. Given that AI-2 communicates growth rate as well as the metabolic burden of recombinant protein synthesis, we postulated that altering the pattern of communication might signal normal growth and cell division rate even in the presence of an applied stress, thus allowing for higher product yields. Plasmid-bearing E. coli cells exposed to conditioned medium, which contained elevated levels (>200-fold) of AI-2, expressed recombinant chloramphenicol acetyltransferase, organophosphorous hydrolase and human interleukin-2 greater than 2.0-, 2.3 and 2.5-fold, respectively, compared to protein-expressing cells grown in conditioned medium lacking AI-2.;Importantly, the enhancing effects of conditioned medium were demonstrated to result from a shift in extracellular AI-2 signaling level. |
