The genes encoding subunits of ATP synthase are conserved in the reduced plastid genome of the heterotrophic alga Prototheca wickerhamii

The heterotrophic unicellular alga Prototheca wickerhamii is closely related to the photoautotrophic Chlorella vulgaris but has a 54.100-bp plastid DNA (ptDNA) that is much smaller than the chloroplast DNA of C. vulgaris (150.613 bp). The nucleotide sequence of 28,093 bp of the Prototheca ptDNA has been determined. No genes for photosynthetic functions have been found. except for sequences encoding six subunits of the ATP synthase (atpA, atpB atpE, atpF. atpH, and atpI). Transcripts of these atp genes have also been detected. Whether the leucoplasts of Prototheca contain a functional ATP synthase has still to be elucidated. Identified genes further include tufA. minD. cysT. and genes coding for three rRNAs. 22 tRNAs. and 12 ribosomal proteins. The results support the idea that. in the reduced plastid genome of Prototheca. genes coding for components of the plastid translational apparatus have been preferentially retained, and might be needed for the expression of the atp genes and some unassigned ORFs.