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Vol. 19, Issue 1, 262-273, January 2008

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The Uni2 Phosphoprotein is a Cell Cycle–regulated Component of the Basal Body Maturation Pathway in Chlamydomonas reinhardtii

Department of Plant Biology, The University of Minnesota, St. Paul, MN 55108

Submitted August 19, 2007; Revised October 4, 2007; Accepted October 10, 2007
Monitoring Editor: Stephen Doxsey

Mutations in the UNI2 locus in Chlamydomonas reinhardtii result in a "uniflagellar" phenotype in which flagellar assembly occurs preferentially from the older basal body and ultrastructural defects reside in the transition zones. The UNI2 gene encodes a protein of 134 kDa that shares 20.5% homology with a human protein. Immunofluorescence microscopy localized the protein on both basal bodies and probasal bodies. The protein is present as at least two molecular-weight variants that can be converted to a single form with phosphatase treatment. Synthesis of Uni2 protein is induced during cell division cycles; accumulation of the phosphorylated form coincides with assembly of transition zones and flagella at the end of the division cycle. Using the Uni2 protein as a cell cycle marker of basal bodies, we observed migration of basal bodies before flagellar resorption in some cells, indicating that flagellar resorption is not required for mitotic progression. We observed the sequential assembly of new probasal bodies beginning at prophase. The uni2 mutants may be defective in the pathways leading to flagellar assembly and to basal body maturation.

Address correspondence to: Carolyn D. Silflow (silfl001{at}umn.edu)

Abbreviations used: CIP, calf alkaline intestinal phosphatase; EST, expressed sequence tag; NFAp, nucleoflagellar apparatus; TAP, Tris-acetate-phosphate medium; TEM, transmission electron microscopy; TR, Texas red; TZ, transition zone; WT, wild type.