The helical mollicute Spiroplasma citri, when growing on low-agar medium, forms fuzzy colonies with occasional surrounding satellite colonies due to the ability of the spiroplasmal cells to move through the agar matrix. In liquid medium, these helical organisms flex, twist, and rotate rapidly. By using Tn4001 insertion mutagenesis, a motility mutant was isolated on the basis of its nondiffuse, sharp-edged colonies. Dark-field microscopy observations revealed that the organism flexed at a low frequency and had lost the ability to rotate about the helix axis. In this mutant, the transposon was shown to be inserted into an open reading frame encoding a putative polypeptide of 409 amino acids for which no significant homology with known proteins was found. The corresponding gene, named scm1, was recovered from the wild-type strain and introduced into the motility mutant by using the S. citri oriC plasmid pBOT1 as the vector. The appearance of fuzzy colonies and the observation that spiroplasma cells displayed rotatory and flexional movements showed the motile phenotype to be restored in the spiroplasmal transformants. The functional complementation of the motility mutant proves the scm1 gene product to be involved in the motility mechanism of S. citri.Spiroplasmas are helical, wall-free bacteria belonging to the class Mollicutes, a group of microorganisms phylogenetically related to gram-positive bacteria with low GϩC contents (48). Besides helical morphology, motility is the most distinctive property of spiroplasmas, especially since these organisms possess no flagella, no periplasmic fibrils, and no axial filaments. Motility of spiroplasmas was first described by Davis and Worley (12) for the corn stunt spiroplasma in extracts of infected plants and was confirmed by Cole and coworkers (9) with cultured cells of Spiroplasma citri, the causal agent of citrus subborn disease. Spiroplasma cells display both rotatory and flexional movements which do not produce translational motility in liquid media of low viscosity. Translational motility could be observed only when viscosity was increased by incorporating agar or methylcellulose in the medium (11). As a result of the ability of spiroplasma cells to move through the agar matrix, spiroplasmas, and in particular S. citri, form diffuse colonies when plated on low-agar medium. Aspects of motility, chemotaxis, and viscotaxis in spiroplasmas have been further documented by Daniels and coworkers (11). On the basis of their work, these authors suggest that translational motility is driven by rotation about the helix axis, whereas flexing of the cell body causes changes of direction. Also from these studies, it is known that motility of spiroplasmas is energy dependent. However, the mechanism of motility has not been elucidated, and the cellular elements involved in motility are still unknown. In order to identify such cellular components, Cohen and coworkers (8) tried to obtain nonmotile mutants of Spiroplasma melliferum BC3. In those studies, nitrous acid was used as the mut...