Isolation and characterisation of chemotactic mutants of Chlamydomonas reinhardtii obtained by insertional mutagenesis

The swimming behaviour of the green flagellated protist Chlamydomonas reinh ardtii is influenced by several different external stimuli including light and chemical attractants. Common components are involved in both the photo- and chemo-sensory transduction pathways, although the nature and organisat ion of these pathways are poorly understood. To learn move about the mechan ism of chemotaxis in Chlamydomonas, we have generated nonchemotactic strain s by insertional mutagenesis. The arginine-requiring strain arg7-8 was tran sformed with DNA carrying the wild-type ARG7 gene. Of the 8630 arginine-ind ependent transformants obtained, five are defective in their chemotaxis tow ards various sugars. Two of the mutants (CTX2 and CTX3) are blocked only in their response to xylose. Mutant CTX1 is blocked in its response to xylose , maltose and mannitol, but displays normal taxis to sucrose. Mutants CTX4 and CTX5 lack chemotactic responses to all sugars tested. CTX1, CTX4 and CT X5 represent novel chemotactic phenotypes not previously obtained using ult ra-violet or chemical mutagenesis. Genetic analysis confirms that each muta tion maps to a single nuclear locus that is unlinked to the mating-type loc us. Further analysis of CTX4 indicates that the mutant allele is tagged by the transforming ARG7 DNA. CTX4 appears to be defective in a component spec ific for chemotactic signal transduction since it exhibits wild-type photob ehavioural responses (phototaxis and photoshock) as well as the wild-type r esponses of EGTA-induced trans-flagellum inactivation and acid-induced defl agellation. Insertional mutagenesis has thus permitted the generation of no vel chemotactic mutants that will be of value in the molecular dissection o il: the signalling machinery.