JOURNAL OF BACrERIOLOGY, Nov. 1992, p. 7048-7052

Vol. 174, No. 21

0021-9193/92/217048-05$02.00/0 Copyright © 1992, American Society for Microbiology

Influence of Carbon Source on Cell Surface Topology of Thermomonospora curvata FRANZ HOSTALKA, ANDREW MOULTRIE, AND FRED STUTZENBERGER* Department of Microbiology, Clemson University, Clemson, South Carolina 29634-1909 Received 8 May 1992/Accepted 14 August 1992

The appearance of cell surface protuberances in Thermomonospora curvata correlated with cell-bound exoenzymes which could be removed by brief sonication. Mycelia grown on cellulose or xylan had numerous protuberances and retained 20 to 25% of endoglucanase and endoxylanase at cell surfaces, while those grown on pectin or starch had few protuberances and negligible bound pectinase or amylase. Biomaterials in nature exist mainly as complex multicomponent polymers such as those in plant cell walls. Studies of bacterial exoenzymes which degrade these polymers have usually employed the soluble proteins found in the cell-free culture fluid (reviewed in reference 13). However, more recent studies (8, 11) have shown that some depolymerizing enzymes of the anaerobe Clostridium thermocellum are retained at the cell surface in discrete complexes termed cellulosomes. Clusters of these complexes (polycellulosomes) are readily visible by scanning electron microscopy as prominent surface protuberances. Cellulosomal components are responsible for both adherence to cellulose and degradation of the polymer to metabolizable soluble sugars (1). This dual role is especially significant in nature, where adherence to a substrate provides competitive advantages by positioning cells for maximal efficiency in both polymer degradation and product uptake. Greater than 90% of cellulose carbon cycling is catalyzed by aerobic microbes (9); therefore, studies of cell surfacebound enzymes on cellulolytic aerobes should be done. Thermomonospora curvata is a good subject for study in this regard; unlike C. thermocellum (which cannot grow on either pectinic substances or xylan [6]), this aerobic, thermophilic actinomycete grows on a wide range of polysaccharides (7, 14, 16, 17) and establishes itself as the dominant bacterial population during the high-temperature decomposition of lignocellulosic materials (5, 12, 15). During growth on cellobiose (a cellulase inducer), T. curvata formed cell surface protuberances (CSP) which appeared similar to the polycellulosomal protuberances of C. thermocellum (3). Here, we show that CSP development in T. curvata is dependent on the polysaccharide carbon source and that it correlates with cell-bound/cell-free exoenzyme ratios. The strain of T. curvata and the conditions for its growth in mineral salts-vitamin minimal medium were as described previously (16). Cotton fibers (Johnson and Johnson surgical grade) used as a cellulose source were ground to pass through a 40-mesh screen. Purified pectin, starch, and xylan were obtained from Sigma Chemical Co. Protein-extracted lucerne fiber, prepared as previously described (18), had a composition (dry weight) of 40% cellulose, 14% hemicellulose, 18% lignin, 14% pectin, and e--ss -

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Influence of carbon source on cell surface topology of Thermomonospora curvata.

The appearance of cell surface protuberances in Thermomonospora curvata correlated with cell-bound exoenzymes which could be removed by brief sonicati...
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