Cytosolic Ca2+ regulates protein expression in E. coli through release from inclusion bodies

The results here are the first clear demonstration of a physiological role for cytosolic Ca2+ in Escherichia coli by releasing a Ca2+ binding protein, apoaequorin, from inclusion bodies. In growth medium LB the cytosolic free Ca2+ was 0.1–0.3 μM. Addition of EGTA reduced this to <0.1 μM, whereas addition of Ca2+ (10 mM) resulted in a cytosolic free Ca2+ of 1–2 μM for at least 2 h. Ca2+ caused a 1.5- to 2-fold increase in the level of apoaequorin induced by IPTG. Whereas EGTA induced a 50% decrease. The effect of a Ca2+ was explained by release of protein from the inclusion bodies, together with a stabilisation of apoaequorin against degradation. Ca2+ also reduced the generation time by 4–5 min. These results have important implications for unravelling the physiological role of cytosolic Ca2+ in bacteria, particularly where several species are competing for the same nutrients, such as in the gut.