Preliminary Communications vanadate ions at the phosphate binding sites does not cause conformational changes in actin necessary for the polymerization at low ionic strength. The identities of the actin regions involved in phosphate and vanadate binding are not known, and further experiments and knowledge of the tertiary structure of actin ~ 9 will be needed for their identification and explanation of a possible role in actin function. Acknowledgement This research was carried out under a grant to P. J. from Muscular Dystrophy Association. S. C. El-Saleh and P. Johnson Department of Chemistry and Colle,qe of Osteopathic Medicine, Ohio University, Athens, Ohio 45701, USA (Received 4 May 1982) References 1 Pollard, T. D. J. Cell Biol. 1981, 91, 156 2 Franzini-Armstrong, C. and Peachey, L. D. J. Cell Biol. 198 l, 91, 166 3 Perry, S. V. Biochem. Soc. Trans. 1977, 7, 593 4 Geisow, M. Nature 1979, 27g, 507 5 Mimura, M. and Asano, A. Nature 1979, 282, 44 6 Waechter, F. and Engel, J. Eur. J. Bioehem. 1977, 74, 227 7 Strzelecka-Golaszewska, H. and Drabikowski, W. Bioehim. Biophys. Acta 1968, 162, 581 8 Barden, J. A. and DosRemedios, C. G. Biochem. Biophys. Res. Commun. 1979, 86, 529 9 Rouayrenc, J.-F. and Travers, F. Eur. J. Biochem. 1981, 116, 73 10 Spudich, J. A. and Watt, S. J. Biol. Chem. 1971, 246, 4866 11 Avissar, N., Kaminsky, E., Leibovich, S. J. and Oplatka, A. Bioehim. Biophys. Acta 1979, 577, 267 12 Maruyama, K. Biochim. Biophys. Aeta 1981, 667, 139 13 Goodno, C. Proc. Natl. Acad. Sci. USA 1979, 76, 2620 14 Perrie, W. T., Smillie, L. B. and Perry, S. V. Bioehem. J. 1973, 135, 151 15 Lowey, S., Slayter, H. S., Weeds, A. G. and Baker, H. J. Mol. Bioh 1969, 42, 1 16 lshikawa, H., Bischofl; R. and Holtzer, H. J. Cell Biol. 1969, 43, 312 17 Johnson, P. and Smillie, L. B. Biochemistry 1977, 16, 2264 18 Rubinson, K. A. Proe. R. So~. Ser. B. 1981, 212, 65 19 Suck, D., Kabsch, W. and Mannherz, H. G. Proc. Natl. Aead. Sei. ~SA 1981, 78, 4319 Fibre-diffraction studies of the extracellular polysaccharide from Pseudomonas elodea X-ray fibre difl?action techniques have been used to probe the structure of the microbial polysaeeharide ]?om Pseudomonas elodea (yellari gum). The polymer adopts a thre~Jbld helical structure with an axial repeat 0.]0.94 nm. The polymerjbrms weak elastic' thermoreversible 9els and on deacetylation .]orms rigid brittle 9els. Deacetylation has been shown to enhariee erystalliriity. Equatorial reflections index onto a tri~lonal unit cell (a = b = 1.64 rim). Keywords: Gelafion; gellan gum; X-ray fibre diffraction: Pseudomonas elodea; extracellular polysaccharide The following report is a preliminary account of fibre diffraction studies of the extracellular polysaccharide produced by the microorganism Pseudomonas elodea. The polymer has been referred to in the literature by the codename PS60, and is also known as gellan gum. Production and recovery conditions, together with preliminary rheological studies, have been described elsewhere~. Gellan gum is claimed to be a linear anionic heteropolysaccharide containing the two neutral sugars glucose and rhamnose in the ratio 2:3, and with an uronic acid content of ~ 22~0 ~. The polymer is acetylated and the O-acetyl content of the clarified product is ~6'/i,. The polysaccharide in unusual in that the rhamnose is apparently 1~, linked and occurs within the polymer backbone ~. The chemical repeat unit structure remains to be determined. Aqueous solutions ofgellan gum are highly viscous and exhibit thixotropic behaviour. At sufficiently high concentrations (>0.05°/0) thermoreversible gelation occurs. Gelation has been observed to be strongly influenced by the type and concentration of cations and the degree of acetylation ~. Deacetylation of the polymer changes the texture of the gels from being weak and elastic to stiff and brittle. These properties make gellan gum a potentially important new polysaccharide for use either as a viscosity enhancing agent, suspending agent or as a replacement for the gelling agents agar or carrageenan. 0141 8130,82/070432 02S03.00 (~) 1982 Butterworth & Co. [Publishers} ktd 432 Int. J. Biol. Macromol., 1982, Vol 4, December An assessment of the industrial potential and a knowledge of the biological role of the polysaccharide requires more detailed information on the chemical and physical structure of the molecules. Molecular models for the gelation of polysaccharides are usually based on X-ray diffraction studies 2 of dried oriented-gel fibres. It is preliminary studies of gellan gum using this technique which are described below. Samples were a gift from A. N. Bennett and A. P. lmeson (Alginate Industries Ltd) in the form of a dried alcohol precipitate from a culture broth. This material was dissolved in dimethyl sulphoxide to form a 1% solution and centrifuged (76000q for 3 h) to remove insoluble matter comprising mostly cell debris, and accounting for ~ 50",,o by mass of the native material. The polysaccharide was recovered by precipitation with ethanol, washed, dissolved in water, dialysed and freeze- dried. Gels were prepared from 13o aqueous solutions and oriented fibres, suitable for X-ray diffraction 3, were produced by stretching dried strips of these gels for about two days at high humidity (RH ~98°~i). X-ray diffraction patterns were recorded under a helium atmosphere and conditions of either high humidity (RH ~ 100°o) or zero humidity. The radiation used was CuK~ and the fibres were dusted with calcite for calibration purposes. Fibre densities were determined by flotation in mixtures of ethanol and carbon tetrachloride. Fiqure la shows a fibre-diffraction pattern obtained from the clarified polymer with an RH of 1003~,. The diffuse equatorial reflections suggest that only poor lateral order of the aligned chains exists. The layer line spacing corresponds to a repeat in the backbone of 2.82 nm. Meridional reflections occur only on layer lines with /= 3n (n, integer) suggesting a three-fold helical structure for the molecule with an axial projection of the chemical repeat unit of 0.94 nm. This value is typical of a [1 +4] diequatorial linked disaccharide unit 4. Partial deacetylation reduced the O-acetyl content from 5.6 to 1.7°0. Figure lb shows the fibre-diffraction