Chemosensory Transduction in Eukaryotic Microorganisms

Signal transduction is a topic that pervades the current literature, and well it should, since it encompasses studies from mitogenesis to synaptic transmis­ sion (1,19,124), Recent years have seen pieces of many cell and physiolog­ ical puzzles fall into place, and the pieces repeatedly are cell surface receptors (30, 130), G proteins (12, 13,49,63,82), second messengers (3, 6, 7, 78b, 90), and protein kinases and phosphatases (33, 43, 58, 65, 72, 97). The excitement of the plot unraveling is being felt in the studies of the chemical senses, in which the primary signals, i.e. ligands for receptors, are chemicals from the outside environment, and the end response can be the sensations of taste or smell in metazoa or the motile or mating responses of unicellular organisms. Here too, at the receptor cell level, the puzzle parts of receptors and signal transduction are being teased out of cells' proteins and genes (16, 24, 67, 78, 105, 141, 147). One chapter in this volume (J. Armitage) describes the intricate detail into which the bacterial chemoresponse system can be dissected (see also 14). Other chapters concentrate on vertebrate taste and insect olfaction. This chapter focuses on chemosensory transduction in eukaryotic microorganisms. These organisms have proven to be useful in making steady progress in the field of chemical sensing, while technical advances are only now allowing the