A Streptomyces rimosus aphVIII gene coding for a new type phosphotransferase provides stable antibiotic resistance to Chlamydomonas reinhardtii.

Although Chlamydomonas reinhardtii serves as the most popular algal model system, no efficient enzymatic selection marker for the nuclear transformation of wild-type cells is available. We sequenced an aminoglycoside 3'-phosphotransferase gene (aph) from Streptomyces rimosus. Though the derived protein sequence is homologous to members of APH type V, it constitutes a new type, named APHVIII. Since the aphVIII gene has a codon bias similar to that of the nuclear genome of green algae, the aphVIII coding sequence was fused to the 5'- and 3'-untranslated regions of the C. reinhardtii rbcS2 gene. C. reinhardtii transformants were capable of inactivating the antibiotics paromomycin, kanamycin, and neomycin, to which wild-type cells are sensitive. After addition of the 5'-region of hsp70A as a second promoter and insertion of the rbcS2 intron I, the transformation rate increased to two transformants per 1 x 10(5) cells, which is close to the efficiency of transforming auxotrophic strains with the homologous marker arg7. Transformation with the promoter-less aphVIII led to random gene fusion at high frequency. In an aphVIII-based reporter gene assay we have found a so far unknown promoter activity of the 3'-untranslated region of rbcS2, that may promote antisense RNA synthesis from the rbcS2 gene in vivo. We conclude that the aphVIII gene is a useful marker for nuclear transformation and promoter tagging of C. reinhardtii wild-type and probably other green algae.