Triskelion #4

The origin of operons, hearing vs. feeling in teams, stationary phase ≠ energy limitation

This is the fourth post in a series called Triskelion. The triskelion is an ancient symbol consisting of a triple spiral (or legs) exhibiting rotational symmetry. The triskelion has different meanings across cultures, but all meanings revolve around a rule of 3’s. In this series, I’ll share three things — a paper, a quote, and a thought. I'll post these whenever I can fill up the three branches of the triskelion. I hope you enjoy.

A Paper

I’ve always been fascinated by the evolution of operon structure. How do two genes that benefit the organism by co-regulation come to be co-regulated? In a new preprint, Kanai, et al. propose that insertion elements are responsible for this by consecutive insertion-deletion-excision reactions. They report their first proof-of-concept experiments to support this.

A Quote

“Players don’t remember what you say in team talks or practices—what they do remember is how you made them feel” –Sir Alf Ramsey

A Thought

I frequently see stationary phase described as an energy-limited state in published papers. Stationary phase is a relative term that describes the lack of net growth in a microbial batch culture. Importantly, although stationary phase can result from energy limitation, it is not defined by energy limitation. And, in the same media, different microbes might enter stationary phase for different reasons.

Cells can enter stationary phase that are not energy limited. For example, a classic tool for microbial physiologists is to limit the growth of cell cultures by excluding sources of nitrogen, phosphorus, vitamins, or other compounds. These cells are in nutrient limited stationary phase, but may not be energy limited. Dr. Hiroshi Nakaido even proposed that E. coli might be limited by Mg2+ in nutrient-rich LB media!

On the flipside, energy limited cells are not necessarily in stationary phase. The classic example of this would be cells cultured in chemostats, where growth rate can be controlled by the supply of energy-yielding substrates. These cells are growing at a constant rate that is limited by the availability of energy, but are not in stationary phase despite displaying no net increase in cell numbers over time.

Thus stationary phase is not the same as energy limitation, and is best thought of as a stress response that primes the cell for tough times ahead (reviewed here).