Organization and sequence of photosynthetic genes from the plastid genome of the holoparasitic flowering plant Cuscuta reflexa

Abstract

We have cloned and sequenced an area of about 6 kb of the plastid DNA (ptDNA) from the holoparasitic plant Cuscuta reflexa. This region contains (in the following order) genes for the cytochrome b6 f-complex subunit V (petG), tRNA(Val) (trnV), tRNA(Met) (trnM), the epsilon- and beta-subunit of the chloroplast ATP-synthase (atpE and atpB) and the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; rbcL). In addition we identified other photosynthesis-related genes (atpA, petB, psaA, psbA, psbB, psbC, and psbD) in C. reflexa by heterologous hybridization. The gene arrangement of the sequenced area is, except for the petG gene, the same as in ptDNAs of other higher plants (e.g. Nicotiana tabacum). Sequence homologies between the Cuscuta genes and corresponding genes from higher plants are in the range of 90%. The only significant difference is that the rbcL gene of C. reflexa encodes a polypeptide which is 18-23 amino acids longer than in other higher plants. This is remarkable since C. reflexa has lost its ability to grow photoautotrophically. The transcript level of the rbcL gene, however, is strongly reduced as compared to tobacco. These findings are compatible with results from Western blotting analysis, where no Rubisco large subunit was detectable, and with the lack of Rubisco activity in crude extracts of C. reflexa.