Plastid ribosomal protein genes from the nonphotosynthetic flagellate Astasia longa.

The plastid genome of the colorless, nonphotosynthetic protist Astasia longa resembles that of the photoautotrophic species Euglena g r a d i s but is only half the size (73 kb instead of 143 kb). A large number of intact ribosomal protein genes, rRNA genes, and tRNA genes have been identified in the 50% of Astasia ptDNA that has been sequenced (Siemeister and Hachtel, 1989; Siemeister et al., 1990a, 1990b). No genes for photosynthetic function have been found except rbcL. A similar loss of photosynthetic genes has occurred from ptDNA of a nonphotosynthetic parasitic flowering plant, Epifagus virginiana (Wolfe et al., 1992). The ptDNA of A. longa is a functional genome, since the large subunit of Rubisco and transcripts of a number of the protein-encoding genes have been detected (Siemeister and Hachtel, 1990). The reduced ptDNA of A. longa must be maintained to express at least one protein with nonphotosynthetic function that is essential for euglenoid flagellates. Here we report on additional genes on the ptDNA of A. longa that encode plastidic ribosomal proteins (Table I). The results corroborate other findings that genes coding for components of the plastid translational apparatus have been specifically retained on the ptDNA of A. longa. We have cloned and sequenced XbaI fragments X6 (4.0 kb) and X 1 1 (2.9 kb) and the 1.5-kb BglII fragment B9 of A. longa ptDNA (for location of these fragments, see Siemeister and Hachtel, 1989). By comparison with the complete sequence of the chloroplast DNA from E. grucilis (EMBL accession No. X70810; Hallick et al., 1993), ribosomal protein genes rpl2, rp120, rp122, rp123, and rpsl9 were identified. rp123 and rpsl9 are split genes in both E. g r a d i s (Christopher et al., 1988) and A. longa. rp123 of A. longa contains two class I11 introns (1 13 and 103 bp in size), whereas three class I11 introns were found in E. gradis rp123 (for classification of introns, see Hallick et al., 1993). Introns 2 and 3 of E. g r a d i s rp123 occur in A. longa rp123 at identical positions, whereas intron 1 is missing in A. longa and exons 1 and 2 of E. gradis rp123 appear to be fused in A. longa. In the rpsl9 gene of both A. longa and E. gracilis, two class I11 introns (101 and 113 bp in A. longa) occur in exactly the same positions. Downstream from the translation stop codons of a11 five ribosomal protein genes, inverted repeat sequences were found that are known to function as efficient RNA processing and stabilizing ele-