Nonlinear relationship between concentration and activity of a bacterial ice nucleation protein.

The expression level of an ice nucleation gene (inaZ) was varied in Escherichia coli to observe the relationship between activity and gene product. The ice nucleation activity increased as the 2nd to 3rd power of the membrane concentration of the inaZ gene product, implying that molecules of InaZ protein interact cooperatively in groups of two to three at the rate-limiting step of ice nucleus assembly. The 2nd to 3rd power relationship was independent of the threshold temperature at which ice nucleation was measured and was consistent over a 500-fold range of protein concentration. Such a relationship indicates that the same rate-limiting step must be common to the formation of ice nuclei displaying all the various threshold temperatures within a bacterial population. Observations of Pseudomonas syringae, expressing the inaZ gene at various levels, were consistent with a similar relationship and hence a similar mechanism of ice nucleus assembly in Pseudomonas.