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 rb cL mRNA in all taxa surveyed to date. In spinach, this structure has b een shown by others to be involved in modulating transcript stability and correct 3' terminus processing, and is a conserved feature of othe r flowering plant rbcL mRNAs. In Chlanydomonas reinhardtii, an analogo us structure has been shown by others to serve as a transcription term inator. Our sequencing data have shown that this region is highly dive rgent in several non-flowering land plants, as evidenced by representa tives from the ferns, conifers, 'fern-allies' and liverworts. To exten d our analysis, a computer-assisted survey of the stem-loop region of the 3' flanking region of published chloroplast rbcL genes was underta ken. The flowering plant rbcL inverted repeats (IR) were remarkably co nserved in sequence, allowing for precise multiple alignments of both monocot and dicot sequences within a single matrix. Surprisingly, sequ ences obtained from non-flowering land plants, algae, photosynthetic p rotists and photosynthetic prokaryotes were extremely variant, in term s of both sequence composition and thermodynamic parameters.