Taking the pulse of flagellum synthesis in a single bacterium

Synthesis and assembly of the bacterial flagellum in E. coli requires the coordinated transcription of over 40 different genes in 14 different operons. Experiments at the population level suggested that the expression of these genes followed a temporal cascade where each operon was sequentially activated in a well-defined order. However, by combining quantitative time-lapse microscopy with a microfluidic device that allows for individual lineage-tracking, we discovered that the dynamics of flagellar transcription in individual bacterium are highly heterogeneous: we found that transcription of flagellar genes fluctuated dramatically in slow stochastic pulses within a single cell, alternating between ‘on’ and ‘off’ periods that span over several generations. By examining the activity of pairs of promoters within the same cell, we find that during some transcriptional pulses only a specific subgroup of flagellar operons are expressed while during other pulses the full 14 operons are expressed simultaneously. We speculate that such pulsating dynamics confers a fitness advantage over continuous gene expression for maintaining a heterogeneous mixture of phenotypes within an isogenic population.