Plastid transcription in the holoparasitic plant genus Cuscuta: parallel loss of the rrn16 PEP-promoter and of the rpoA and rpoB genes coding for the plastid-encoded RNA polymerase.

The holoparasitic plant genus Cuscuta comprises a range of species whose plastid genomes have different degrees of reductions in their coding capacity. In this study, four Cuscuta species, Cuscuta reflexa, C. gronovii, C. subinclusa and C. odorata, that possess substantial physiological differences, were analysed with respect to the sequence and promoter structure of the rrn16 gene coding for the ribosomal 16S rRNA. Whereas the coding region of this gene is highly conserved among all four Cuscuta species, significant differences were observed in the non-coding region 5' of rrn16 with respect to both the length of the intergenic region between rrn16 and trnV and the promoters used to initiate transcription of the rrn16 gene. In the green species C. reflexa, rrn16 transcription starts from a functional plastid-encoded RNA polymerase (PEP) promoter that is missing in the other three species, C. gronovii, C. odorata and C. subinclusa. Instead, a 15-nucleotide-long conserved sequence immediately upstream of the mapped 5' ends bearing the nuclear-encoded RNA polymerase (NEP) promoter motif could be identified in these three species. The lack of a PEP promoter in these species coincides with the loss of two genes that encode subunits of PEP (rpoA and rpoB).