text: In bacteria, where two-component signaling is extremely common, about 25% of known histidine kinases are of the hybrid type. Two-component systems are much rarer in archaea and eukaryotes, and occur in lower eukaryotes and in plants but not in metazoans. Among known examples, most if not all eukaryotic two-component systems are hybrid kinase phosphorelays.
  A bioinformatic census of bacterial genomes found large variations in the number of (monomeric) HPt domains identified in different bacterial phyla, with some genomes encoding no HPts at all. Relative to the number of histidine kinase and response regulators present in a genome, eukaryotes have more identifiable HPt domains than bacteria. In fungi, the genomic inventory of HPt proteins varies, with filamentous fungi generally possessing more HPt proteins than yeasts; only one is encoded in the well-characterized Saccharomyces cerevisiae genome. Plants generally have more than one HPt, but fewer HPts than response regulators.