The rbcL genes of two Cuscuta species, C. gronovii and C. subinclusa, are transcribed by the nuclear-encoded plastid RNA polymerase (NEP)

Abstract

Some species of the holoparasitic flowering plant genus Cuscuta, like C. reflexa, have retained a plastid genome that encodes photosynthesis-related gene products as well as the plastid-encoded RNA polymerase (PEP). In contrast, other species like C. gronovii and C. subinclusa have lost the rpo genes coding for the PEP subunits while photosynthetic genes have been retained. In order to ensure expression of the photosynthesis-related genes in the absence of PEP, a number of adaptations within the plastid genome were required that enable gene transcription mediated exclusively by the nuclear-encoded plastid RNA polymerase (NEP). In this study we analyzed promoter sequence conservation and transcription start sites of a typical PEP gene of non-parasitic plants, rbcL, which codes for the large subunit of ribulose bisphosphate carboxylase/oxygenase. We show that despite high sequence conservation of the coding region of rbcL among different Cuscuta species and tobacco, the 5′ non-coding regions of C. gronovii and C. subinclusa have suffered extensive deletions encompassing the PEP promoter that is present in C. reflexa and tobacco. Primer-extension analyses enabled the identification of transcripts initiated at NEP promoter motifs in C. gronovii and C. subinclusa that are not detectable in the 5′ non-coding region of C. reflexa.