The sliding microtubule model of ciliary motility predicts that cumulative local displacement (A/) of doublet microtubules relative to one another occurs only in bent regions of the axoneme. We have now tested this prediction by using the radial spokes which join the A sub fiber of each doublet to the central sheath as markers of microtubule alignment to measure sliding displacements directly. Gill cilia from the mussel Elliptio complanatus have radial spokes lying in groups of three which repeat at 860 A along the A subfiber. The spokes are aligned with the two rows of projections along each of the central microtubules that form the central sheath. The projections repeat at 143 A and form a vernier with the radial spokes in the precise ratio of 6 projection repeats to 1 spoke group repeat. In straight regions of the axoneme, either proximal or distal to a bend, the relative position of spoke groups between any two doublets remains constant for the length of that region. However, in bent regions, the position of spoke groups changes systematically so that Al (doublet l vs. 5) can be seen to accumulate at a maximum of 122 A per successive 860-A spoke repeat. 'Local contraction of microtubules is absent. In straight regions of the axoneme, the radial spokes lie in either of two basic configurations: (a) the parallel configuration where spokes 1-3 of each group are normal (90 °) to subfiber A, and (b) the tilted spoke 3 configuration where spoke 3 forms an angle (0) of 9-20 °. Since considerable sliding of doublets relative to the central sheath (~650 A) has usually occurred in these regions, the spokes must be considered, functionally, as detached from the sheath projections. In bent regions of the axoneme, two additional spoke configurations occur where all three spokes of each group are tilted to a maximum of ± 33 ° from normal. Since the spoke angles do not lie on radii through the center of bend curvature, and Al accumulates in the bend, the spokes must be considered as attached to the sheath when bending occurs. The observed radial spoke configurations strongly imply that there is a precise cycle of spoke detachment-reattachment to the central sheath which we conclude forms the main part of the mechanism converting active interdoublet sliding into local bending.

1977. Characterization of SPP: a temperate bacteriophage fromBocillrissrrbtilis 168M. Can. J . Microbial. 23: 45-51. Bocillrrs srrbtilis 168M is lysogenic for a temperate bacteriophage called SPP. The virus head is 76 nm wide by 82 nm long and the tail measures 12 by 358 nm. The DNA molecular weight is 62 million. SPP is spontaneously released at low levels in cultures of B. .slrbtilis 168M, and can be induced at higher levels by treatment with mitomycin C or N-methyl-N'-nitro-N-nitrosoguanidine.

The sperm flagella of the blowfly Sarcophaga bullata demonstrate the relationship of radial projections in the matrix region to the microtubule organization of the axoneme. The A microtubule of each peripheral doublet is connected to the central sheath by a series of paired radial links. The links lie along the tubule wall with a alternate spacing of about 320/560 A. The distal end of each link is enlarged into a globular head that connects via a transitional link to the helical sheath around the central microtubules. The radial link pairs are disposed in the form of a double helix with a pitch of about 1760 A. It is proposed that a similar organization is common to all cilia and flagella showing ninefold symmetry and must provide, in part, the morphological basis for motility.

The sliding tubule model of ciliary motion requires that active sliding of microtubules occur by cyclic cross-bridging of the dynein arms. When isolated, demembranated Tetrahymena cilia are allowed to spontaneously disintegrate in the presence of ATP, the structural conformation of the dynein arms can be dearly resolved by negative contrast electron microscopy. The arms consist of three structural subunits that occur in two basic conformations with respect to the adjacent B subfiber. The inactive conformation occurs in the absence of ATP and is characterized by a uniform, 32 ~ base-directed polarity of the arms. Inactive arms are not attached to the B subfiber of adjacent doublets. The bridged conformation occurs strictly in the presence of ATP and is characterized by arms having the same polarity as inactive arms, but the terminal subunit of the arms has become attached to the B subfiber. In most instances the bridged conformation is accompanied by substantial tip-directed sliding displacement of the bridged doublets. Because the base-directed polarity of the bridged arms is opposite to the direction required for force generation in these cilia and because the bridges occur in the presence of ATP, it is suggested that the bridged conformation may represent the initial attachment phase of the dynein cross-bridge cycle. The force-generating phase of the cycle would then require a tip-directed deflection of the arm subunit attached to the B subfiber.KEY WORDS motility cilia microtubules cross-bridges dynein 9 ATP Several independent studies confirm the sliding tubule mechanism of ciliary and flagellar motion (20,21,25); however, we as yet do not understand how the adenosine triphosphatase or dynein arms actually participate in the mechanochemical activity that must be responsible for linear displacement of microtubules. The displacement event clearly involves both the enzyme-mediated hydrolysis of ATP and the formation by the arms of intermittent cross-bridges between adjacent doublets (10, 12, 13). However, the relationship of the chemical events to the structural or molecular events is not understood. It is, of course, necessary that the dynein cross-bridges have the capability to alter their orientation in the bridged condition in order to cause sliding. The dynein arms were recently found to consist of 3-4 morphologically similar subunits that in isolated form have a mean diameter of 93 .~ and electrophorese as dynein 1 (26). The subunits, as a polymer, presumably have both an active site that can bind B subfiber tubulin and a region of conformational instability that will, in the correct