Ca2+ signaling in the Chlamydomonas flagellar regeneration system: cellular and molecular responses.

In response to certain extracellular stimuli, Chlamydomonas reinhardtii cells excise their flagella, induce expression of more than 200 different flagellar mRNAs, and assemble a new flagellar pair. Normally, flagellar excision, gene induction and outgrowth are tightly coupled temporally. Our previous studies showed that uncoupling the cellular response of flagellar excision from flagellar outgrowth resulted in submaximal flagellar gene induction, and led us to propose that normal flagellar gene induction is a composite response. The present study extends these observations by measuring flagellar gene induction in Chlamydomonas cells stimulated under conditions where both flagellar excision and flagellar outgrowth are blocked. We find that the flagellar genes are induced in a Ca(2+)-dependent manner in response to stimulation in the absence of flagellar excision and outgrowth. Flagellar gene induction is therefore independent of flagellar excision and outgrowth but sensitive to extracellular Ca2+ levels. Thus, flagellar excision, flagellar outgrowth and flagellar gene induction are three responses to a common stimulus that are related by their requirement for extracellular Ca2+.