Promoter Shuffling at a Nuclear Gene for Mitochondrial RPL27. Involvement of Interchromosome and Subsequent Intrachromosome Recombinations

The Reclinomonas americana mitochondrial genome contains a mitochondrial ribosomal protein L27 (rpl27) gene, whereas the rpl27 gene is absent from all plant mitochondrial genomes examined to date. This suggests that plant mitochondrial rpl27 genes have been transferred previously from the mitochondrial genome to the nuclear genome. A nuclear cDNA encoding mitochondrial RPL27 was identified in rice (Oryza sativa). Three similar sequences were identified: rpl27-1 and rpl27-2 on chromosome 8 and rpl27-3 on chromosome 4. Harr plot analysis suggests that they were generated by inter- and intrachromosomal duplications. Interestingly, the transcribed rpl27 gene (rpl27-1) acquired a promoter sequence that was derived from the rice spt16 (Osspt16) gene, the homolog of a global transcription factor in yeast (Saccharomyces cerevisiae) located downstream from the rpl27-3 sequence on chromosome 4, after inter- and intrachromosomal recombination. Reverse transcription-PCR and promoter assay revealed that the rpl27 mRNAs were mainly transcribed from rpl27-1. A repeat of seven nucleotides (AATAGTT) was identified at the junction of rpl27-1 and rpl27-2 on chromosome 8, and the same repeat was also identified at the 5' end of rpl27-2 and the 3' end of rpl27-1. This repeat (AATAGTT) contains the hot-spot sequence AGTT, which is preferentially recognized by topoisomerase I in wheat (Triticum aestivum) germ, suggesting the involvement of topoisomerase I in this recombination. We here report the example of promoter shuffling and show that this promoter shuffling resulted from a recent segmental duplication through inter- and intrachromosomal recombination events.