Abstract
In chloroplasts, the control of mRNA stability is of critical importance for proper regulation of gene expression. The Chlamydomonas reinhardtii strain Δ26pAtE is engineered such that the atpB mRNA terminates with an mRNA destabilizing polyadenylate tract, resulting in this strain being unable to conduct photosynthesis. Spa19 and spa23 were photosynthetic revertants and they maintained unusual heteroplasmic chloroplast genomes (PS+ and PS-). Based on strand-specific RT-PCR, S1 nuclease protection, and RNA gel blots, evidence was obtained that the PS+ genome stabilizes atpB mRNA by generating an atpB antisense transcript. To obtain additional evidence for antisense RNA function in chloroplasts, a segment of atpB antisense RNA was expressed from an ectopic site, which resulted in an elevated accumulation of atpB mRNA. Taken together, our results suggest that antisense RNA in chloroplasts can protect otherwise unstable transcripts from 3'->5' exonuclease activity, a phenomenon that may occur naturally in the symmetrically transcribed chloroplast genome.
