Chloroplast transformation in Euglena gracilis:: splicing of a group III twintron transcribed from a transgenic psbK operon

The Escherichia coli aadA gene product, which confers resistance to spectinomycin and streptomycin, has been widely used as a dominant selectable marker for chloroplast transformation of Chlamydomonas and tobacco. An aadA transformation cassette was adapted for expression in Euglena gracilis chloroplasts by replacing the Chlamydomonas promoter and 3' untranslated region (UTR) with the E. gracilis psbA promoter and 3' UTR. Transgenic DNA was introduced into E. gracilis chloroplasts by biolistic transformation. Streptomycin- and spectinomycin-resistant colonies were obtained, which screened positively for the presence of the transforming vector by PCR amplification. Although integration of the transforming DNA into the chloroplast genome was not detected, transforming DNA was stably maintained in the chloroplast as an episomal element during continuous selection on antibiotics. The aadA cassette was also inserted into a transformation vector which contained the independently expressed psbK operon from either E. gracilis or a closely related species, E. stellata. The psbK operon contained at least: two group III introns and a group III twintron, was highly expressed, and was only 1.5 kb in length. In transgenic E. gracilis chloroplasts, a truncated E. stellata psbK operon was transcribed, and the resultant pre-mRNA was accurately spliced. This system should allow the first direct analysis of group II and group III intron-splicing mechanisms. In addition, it could prove useful in the study of many other Euglena transcription and processing events.