Kinetics of appearance and disappearance of classes of bacterial ice nuclei support an aggregation model for ice nucleus assembly.

The kinetics of appearance and disappearance of three classes of ice nuclei in Pseudomonas syringae was investigated under conditions where high-level expression of the ice nucleation phenotype was obtained. The appearance of types 1, 2, and 3 ice nuclei, catalyzing nucleation at -2 to -5, -5 to -7, and -7 to -10 degrees C, respectively, was investigated during low-temperature induction in wild-type strains and in a unique, detergent-sensitive mutant that contained no type 3 ice nuclei when grown at 32 degrees C. Nuclei appeared in the following order: type 3, then type 2 and type 1. The disappearance of classes of ice nuclei was monitored during high-temperature treatment of fully induced cells; nuclei disappeared in the order type 1, type 2, and type 3. Although analysis of nucleation events is complicated by masking and unmasking of ice sites in the same cells, these temporal sequences of ice nucleus appearance or disappearance are consistent with an aggregation model for ice nucleus assembly (A. G. Govindarajan and S. E. Lindow, Proc. Natl. Acad. Sci. USA 85:1334-1338, 1988; G. Warren and P. Wolber, Mol. Microbiol. 5:239-243, 1991).