A Chlamydomonas Outer Arm Dynein Mutant Missing the te Heavy Chain

A novel Chlamydomonas flagellar mutant (oda-11) missing the ot heavy chain of outer arm dynein but retaining the/3 and 3~ heavy chains was isolated. Restriction fragment length polymorphism analysis with an ct heavy chain locus genomic probe indicated that the oda-11 mutation was genetically linked with the structural gene of the tx heavy chain. In crosssection electron micrographs, the oda-ll axoneme lacked the outermost appendage of the outer arm, indicating that the a heavy chain should be locate¢[ in this region in the wild-type outer arm. This mutant swam at 119 #m/s at 25°C, i.e., at an intermediate speed between those of wild type (194/~m/s) and of oda-1 (62/zm/s), a mutant missing the entire outer dynein arm. The flagellar beat frequency (,x,50 Hz) was also between those of wild type (~,60 Hz) and oda-1 (,x,26 Hz). These results indicate that the outer dynein arm of Chlamydomonas can be assembled without the ot heavy chain, and that the outer arm missing the e~ heavy chain retains partial function. T HE outer arm dynein of cilia and flagella has been the most extensively studied among all kinds of dynein. Chlamydomonas outer arm dynein contains three heavy chains (a,/3, and 3'), which have molecular masses in excess of 400,000 and ATPase activity, as well as several small and intermediate-sized subunits (Piperno and Luck, 1979; Huang et al., 1979; Pfister et al., 1982; King and Witman, 1988a,b). Tetrahymena outer arm dynein also has three heavy chains (Johnson and Wall, 1983; Toyoshima, 1987). On the other hand, outer arm dyneins from sperm flagella of sea urchin (Bell and Gibbons, 1982; Bell, 1983; Sale et al., 1985), trout (Gatti et al., 1989), and bull (MarcheseRagona et al., 1987), and those from cilia of sea urchin embryo (Ogawa et al., 1990) contain only two heavy chains. Electron microscope studies have shown that the isolated outer arms have bouquet-like structures in which two or three heads are connected through a thin stem to a common base (Johnson and Wail, 1983; Witman et at., 1983; Goodenough and Heuser, 1984; Sale et al., 1985). Each of these heads appears to be composed of a heavy chain, because the number of the heads equals that of the heavy chains. It is not known, however, how these bouquet-like outer arms are folded when attached to the A-tubule of the outer doublets. It is not known either how the two-headed and three-headed dyneins differ in function, nor what specific function each heavy chain performs. Recent experiments using purified dynein subspecies have demonstrated that a partial assembly of sea urchin outer arms containing only one heavy chain (Sale and Fox, 1988; Vale et al., 1989), or that of Tetrahymena outer arms containing only two heavy chains (Vale and Toyoshima, 1989) can cause translocation of cytoplasmic microtubules in vitro. Thus partial outer arms retain function as mechanochemical transducers. It has remained to be studied whether these partial outer arms can function in beating axonemes. In this paper we describe the isolation of an unusual Chlamydomonas mutant that lacks the o~ heavy chain of the outer arm. Because this mutant can swim at an intermediate speed between the speeds of the wild type and a mutant (oda) missing the entire outer arm, the mutant outer arm appears to retain some functional activity. Materials and Methods