SUMMARY An adaptation of the ‘correlation averaging’ method is described which allows reliable and almost fully automatic image averaging in the case of near‐periodic structures notwithstanding the presence of substantial crystal imperfections; methods for assessing resolution and symmetry without reliance on crystallinity are also discussed. Electron micrographs of negatively stained and rotary shadowed preparations of the HPI‐layer protein from the cell envelope of Micrococcus radiodurans have been averaged using the method, and the projected structure is described to a resolution of about 1·9 nm.

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The cell walls of four gliding filamentous Oscillatoriaceae species comprising three different genera were studied by freeze substitution, freeze fracturing, and negative staining. In all species, the multilayered gramnegative cell wall is covered with a complex external double layer. The first layer is a tetragonal crystalline S-layer anchored on the outer membrane. The second array is formed by parallel, helically arranged surface fibrils with diameters of 8 to 12 nm. These fibrils have a serrated appearance in cross sections. In all cases, the orientation of the surface fibrils correlates with the sense of revolution of the filaments during gliding, i.e., clockwise in both Phormidium strains and counterclockwise in Oscillatoria princeps and Lyngbya aeruginosa. The lack of longitudinal corrugations or contractions of the surface fibrils and the identical appearances of motile and nonmotile filaments suggest that this structure plays a passive screw thread role in gliding. It is hypothesized that the necessary propulsive force is generated by shear forces between the surface fibrils and the continuing flow of secreted extracellular slime. Furthermore, the so-called junctional pores seem to be the extrusion sites of the slime. In motile cells, these pores exhibit a different staining behavior than that seen in nonmotile ones. In the former, the channels of the pores are filled with electron-dense material, whereas in the latter, the channels appear comparatively empty, highly contrasting the peptidoglycan. Finally, the presence of regular surface structures in other gliding prokaryotes is considered an indication that comparable structures are general features of the cell walls of gliding microbes.Many multicellular, filamentous cyanobacteria move on solid surfaces by a type of motility described as gliding. Gliding is not exclusive to cyanobacteria but rather is widespread among prokaryotes (for a review, see references 3 and 20). It is unknown whether gliding motility is in all cases based on a common mechanism.Gliding of the nonpolar, filamentous cyanobacteria appears as a slow uniform motion, occasionally interrupted by reversals (13). Neither wriggling nor contraction nor peristaltic alterations of the filamentous trichomes are detectable by light microscopy (38). Members of the Oscillatoriaceae translocate in a highly coordinated manner. Translational movements are accompanied in most species by revolution around the long axis of the filament (4). While moving, the trichomes secrete slime which is left behind as a twisted and collapsed thin tube (20). The sense of revolution is unique and species specific (46), as evinced by the handedness of the rotation.The lack of flagella or of other plausible locomotive organelles is puzzling. Electron microscopic studies have so far failed to identify motor structures common among gliders. A number of unusual structures have been described which may or may not be related to this peculiar type of motility. This list comprises spinning discs (37), fibrils within the ce...