A specific and highly potent inhibitor of diguanylate cyclase, the key regulatory enzyme of the cellulose synthesizing apparatus in the bacterium Acetobacter xylinum, was isolated from extracts of etiolated pea shoots (Pisum sativum). The inhibitor has been purified by a multistep procedure, and sufficient amounts of highly purified compound (3-8 mg) for spectral analysis were obtained. The structure of this compound was established as 3-O-alpha-L-rhamnopyranosyl-(1-->2)-beta-D-galactopyranosyl-(1--> 2)-beta-D-glucuronopyranosyl soyasapogenol B 22-O-alpha-D-glucopyranoside. The structure was elucidated on the basis of susceptibility to various enzymes, chemical and spectral methods, such as GC-MS, FAB-MS, and the following types of 2D-NMR: COSY, ROESY, TOCSEY, HMQC, HMBC analyses. An identical or a very similar compound with identical biological activity was also isolated from A. xylinum, strongly suggesting that at least certain aspects of cellulose synthesis in the bacteria and in higher plants may be regulated in a similar manner. The content of this saponin in etiolated plants was about 0.04 mumol (g fresh tissue)-1.