Chlamydomonas reinhardtii Tubulin-Gene Disruptants for Efficient Isolation of Strains Bearing Novel Tubulin Mutations

Abstract

The single-cell green alga Chlamydomonas reinhardtii possesses two -tubulin genes (tua1 and tua2) and two {beta}-tubulin genes (tub1 and tub2), with the two genes in each pair encoding identical amino acid sequences. Here, we used an aphVIII gene cassette insertional library to establish eight disruptants with defective tua2, tub1, or tub2 expression. None of the disruptants exhibited apparent defects in cell growth, flagellar length, or flagellar regeneration after amputation. Because few tubulin mutants of C. reinhardtii have been reported to date, we then used our disruptants, together with a tua1 disruptant obtained from the Chlamydomonas Library Project (CLiP), to isolate novel tubulin-mutants resistant to the anti-tubulin agents propyzamide and oryzalin. As a result of several trials, we obtained 8 strains bearing 7 different -tubulin mutations and 24 strains bearing 12 different {beta}-tubulin mutations. Some of these mutations are known to confer drug resistance in human cancer cells. Thus, single-tubulin-gene disruptants are an efficient means of isolating novel C. reinhardtii tubulin mutants. IMPORTANCEChlamydomonas reinhardtii is a useful organism for the study of tubulin function; however, only five kinds of tubulin mutations have been reported to date. This scarcity is partly due to C. reinhardtii possessing two tubulin genes each for - and {beta}-tubulin. Here, we obtained several strains in which one of the - or {beta}-tubulin genes was disrupted, and then used those disruptants to isolate 32 strains bearing 19 mostly novel tubulin mutations that conferred differing degrees of resistance to two anti-tubulin compounds. The majority of the tubulin mutations were located outside of the drug-binding sites in the three-dimensional tubulin structure, suggesting that structural changes underlie the drug resistance conferred by these mutations. Thus, single-tubulin-gene disruptants are an efficient means of generating tubulin mutants for the study of the structure-function relationship of tubulin and for the development of novel therapies based on anti-tubulin agents.