STRUCTURAL AND FUNCTIONAL-ANALYSIS OF PLASTID GENOMES FROM PARASITIC PLANTS - LOSS OF AN INTRON WITHIN THE GENUS CUSCUTA

The gene cluster rps12/rps7/psi ndhB of the plastome from the holopara sitic plant Cuscuta europaea has been analysed at the nucleotide level . A comparison with the homologous region of the plastome from the clo sely related parasite Cuscuta reflexa reveals a complete loss of the c is-spliced intron of the rps12 gene in addition to a drastic size redu ction of the ndhB pseudogene. It is demonstrated by RT-PCR analysis th at the entire gene cluster is transcribed in the form of a multicistro nic transcript which also includes the sequences encoded by the ndhB p seudogene. A cDNA containing the correctly trans-spliced exon 1 of the rps12 transcript can also be amplified. This shows that trans-splicin g of the rps12 transcript persists in the plastids of the holoparasite despite the loss of the cis-spliced intron and the loss of many other gene functions. The rps12 and rps7 genes, therefore, still appear to code for functional ribosomal proteins CS12 and CS7, respectively. The conservation of apparently intact ribosomal-protein genes from which correctly processed transcripts are produced is taken as evidence that the translational apparatus of the plastids is still functional and n ecessary for the expression of the genes remaining in the reduced plas tome of a parasitic plant.