The symbolic power of nucleotide second messengers - or how prokaryotes link sensing and responding to their outside world.

Editorial Nucleotide second messengers are k e y components of molecular information processing pathways and networks that allow bacteria and archaea to navigate and adapt to an e v er c hanging exter-nal world. Being produced and/or degraded by often membrane-associated enzymes that can sense and react to environmental or cellular changes, the intracellular second messenger molecules are in turn sensed by cellular effectors—usually proteins or protein domains , sometimes ribos witc hes—that trigger dir ectl y associated target systems to produce specific molecular reactions. T hus , second messengers are informational molecules, which stand as intracellular molecular symbols or signs for something else, namel y, some potentiall y life-thr eatening condition, and which inform the executive machinery of the cell that an a ppr o-priate ada ptiv e r esponse is ur gentl y needed. It is pr obabl y not accidental that two major classes of intracellular molecules with symbolic functions are both deri vati ves of n ucleotides although there is a clear functional division of labour between them. The nucleotide polymers DNA and RNA function in the stor a ge and ef-fectuation of genetic information that has to be maintained over long times. By constrast, nucleotide second messengers are tiny RNAs, which consist only of one or two, often cyclic nucleotides, that have to transiently represent environmental or cellular states in real time . T his requires high dynamics of second messenger pr oduction and degr adation by specific enzymes whose expr es-sion and activity has to be tightl y contr olled by sensory input. It is likely that second messenger signaling is the evolutionary oldest form of molecular information processing, rudimentary forms of which ma y ha ve already evolved in the early RN A w orld. The cur-r ent pictur e of second messenger signaling in Pr okary otes, ho we