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Regulating Pilin Expression Reveals a Threshold for S Motility in Myxococcus xanthus

Lotte Jelsbak and Dale Kaiser^*

Departments of Biochemistry and Developmental Biology, Stanford University, Stanford, California

Received 17 August 2004/ Accepted 23 November 2004

An isopropyl-ß-D-thiogalactopyranoside (IPTG)-inducible promoter was constructed in Myxococcus xanthus. The single-copy pilA gene encodes pilin, the monomer unit of M. xanthus type IV pili. To vary the level of pilA expression, we cloned its promoter in front of the lac operator, and a plasmid containing the construct was inserted into the chromosome of a pilA strain. Induction of pilin expression increased smoothly as the dose of IPTG added to the culture was increased. IPTG-induced pilin rescued S motility of the pilA strain to wild-type levels. The rate of S-motile swarming was found to be proportional to the number of pili (shear-sensitive pilin) produced rather than to the level of total pilin. In fact, S motility was not rescued until the total level of pilin was more than 50% of the wild-type level. This observation implies that a threshold concentration of pilin must be exceeded before the shear-sensitive material (pili) is polymerized in M. xanthus.

Present address: Department of Veterinary Pathology, Royal Veterinary and Agricultural University, DK-1870 Frederiksberg C, Denmark.

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