exhibits an unusual flagellation pattern in the electron microscope. The typical pattern is of two flagella in a polar or sub-polar tuft at one pole only, plus one or two lateral flagella on each side. Twenty independent behavioural mutants were isolated, after Tn5 mutagenesis, and divided into several classes. Fourteen non-motile mutants were differentiable into non-flagellate and flagellate subclasses, one non-chemotactic mutant appeared to have a defect in the intracellular signalling pathway, and one other mutant displayed an unusual pattern of polar cell pairing. Four mutants with impaired swarming ability were also isolated. Root colonization ability was impaired in the non-motile mutant for which this property was tested. From all but three of the mutants, the Tn5 insertion sites, including flanking sequences, have been cloned and used as probes to isolate a set of seven cosmids from a wild-type library. No hybridization to or DNA was detected for the mutant flanking sequences, but varying degrees of similarity to behavioural genes were detected. By complementation of behavioural mutants and hybridization of mutant flanking sequences, one cosmid insert was shown to contain at least nine behavioural loci, and another, linked to it on the chromosome, at least three. This suggests that, as in other organisms, behavioural genes are clustered in the genome.


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