THE RBCL GENE FROM THE NONPHOTOSYNTHETIC PARASITE LATHRAEA-CLANDESTINA IS NOT TRANSCRIBED BY A PLASTID ENCODED RNA-POLYMERASE

In the plastome of the obligate root-parasitic plant, Lathraea clandes tina, the rbcL gene has been maintained and is expressed, despite the reduced size and gene content of the plastid genome. Some of the plast id genes involved in translation (e.g. transfer RNAs, ribosomal RNAs a nd ribosomal proteins) have been sequenced and still appear to code fo r functional ribosomal components. Indeed, the 16S rRNA and rpl20 gene s are expressed whilst other necessary tRNA and ribosomal protein-enco ding genes have probably been deleted or truncated. Although obtained by PCR, the four rpo genes for Escherichia coli-like plastid encoded R NA polymerase appear to be pseudogenes. Nevertheless, the rbcL gene, w ith a ''-10, -35'' prokaryotic-like promoter, is still transcribed. In contrast to photosynthetic plants, rbcL transcripts in Lathraea are l arger in their 5' region and cover the prokaryotic-like promoter. The transcription initiation site is located near the ATG start codon of t he atpB pseudogene. Similarity to nonconsensus E. coli-like plastid pr omoters suggests that rbcL transcription is driven by a nuclear-encode d RNA polymerase.