EXTENSIVE SEQUENCE DIVERGENCE IN THE 3' INVERTED REPEAT OF THE CHLOROPLAST RBCL GENE IN NONFLOWERING LAND PLANTS AND ALGAE

A stem-loop region is present at the 3' terminus of the chloroplast rbcL mRNA in all taxa surveyed to date. In spinach, this structure has been shown by others to be involved in modulating transcript stability and correct 3' terminus processing, and is a conserved feature of other flowering plant rbcL mRNAs. In Chlanydomonas reinhardtii, an analogous structure has been shown by others to serve as a transcription terminator. Our sequencing data have shown that this region is highly divergent in several non-flowering land plants, as evidenced by representatives from the ferns, conifers, 'fern-allies' and liverworts. To extend our analysis, a computer-assisted survey of the stem-loop region of the 3' flanking region of published chloroplast rbcL genes was undertaken. The flowering plant rbcL inverted repeats (IR) were remarkably conserved in sequence, allowing for precise multiple alignments of both monocot and dicot sequences within a single matrix. Surprisingly, sequences obtained from non-flowering land plants, algae, photosynthetic protists and photosynthetic prokaryotes were extremely variant, in terms of both sequence composition and thermodynamic parameters.