Chemosensory transduction is an important function in cells ranging from prokaryotes to differentiated eukaryotic tissues such as neurons. The functions of chemoreception are quite diverse, vary in accordance with cell type, and range from finding food sources in Tetrahymena and escape from predators in Paramecium to directing patterns of neural growth in mammalian systems.

The chemorepellent used in this study, hen egg lysozyme, is an example of a Class I repellent in Paramecium. It has been shown to be a secretagogue as well as a repellent in Paramecium, possibly mimicking the effects of compounds found in predator exudates.

In this study, we have attempted to shed light on the mechanism of lysozyme action in Paramecium as well as in the related ciliate, Tetrahymena thermophila. We have found that both ciliates respond to micromolar quantities of lysozyme by means of an avoidance reaction which is correlated with a depolarization, both ciliates bind lysozyme with a single class of binding sites, and finally, both species reversibly adapt to lysozyme over a ten minute time period. The receptors have been purified from both ciliate species, and a number of differences may be noted. Firstly, the receptor from Paramecium consists of a single, 58 kD protein, while that of Tetrahymena is a single 44 kD protein. Secondly, polyclonal antibodies raised against the lysozyme receptor in Tetrahymena, while blocking behavioral avoidance in this organism, fail to block lysozyme avoidance in Paramecium or to recognize the Paramecium receptor in an ELISA assay. These data point to differences in the mechanism of lysozyme reception between the two species.

Finally, we have found that the entire structure of lysozyme is not required in order to elicit avoidance. CB$\sb2,$ a 24 amino-acid fragment of lysozyme, has been found to elicit behavioral avoidance in Tetrahymena at even lower concentrations than does intact lysozyme, and causes a transient depolarization which is similar to that seen with intact lysozyme.

Much work remains to be done in order to determine how lysozyme adaptation occurs as well as what amino acids are critical in eliciting avoidance behavior. The current work should be foundational to these future studies.