INTRODUCTION OF A HETEROLOGOUS EDITING SITE INTO THE TOBACCO PLASTID GENOME - THE LACK OF RNA EDITING LEADS TO A MUTANT PHENOTYPE

The psbF mRNA is edited in spinach plastids by a C to U conversion, ch anging a serine to a conserved phenylalanine codon. In tobacco at this position a phenylalanine codon is present at the DNA level, and the p sbF mRNA here is not edited. To test if the psbF editing capacity is e volutionarily conserved, the tobacco psbF gene was modified to match t he corresponding spinach sequence. The endogenous tobacco gene was rep laced with the modified copy using biolistic transformation. We report here that the heterologous editing site remains unmodified in transpl astomic tobacco plants. The lack of editing is associated with slower growth, lowered chlorophyll content and high chlorophyll fluorescence, a phenotype characteristic of photosynthetic mutants. This finding co nfirms that the editing of the psbF mRNA is an essential processing st ep for protein function and thus provides direct proof for the biologi cal significance of plant organellar RNA editing. Given that a mutant phenotype is associated with the lack of editing, it seems likely that the evolutionary loss of the site-specific capacity for psbF editing was preceded by the mutation that eliminated the editing requirement.