A chloroplast transcript lacking the 3' inverted repeat is degraded by 3'-->5' exoribonuclease activity

Chlamydomonas reinhardtii strains harboring deletions of the chloroplast atpB 3' inverted repeat (IR) are weakly phototrophic due to reduced accumulation of discrete atpB transcripts and the chloroplast ATPase beta-subunit protein. A sequence of 18 guanosine residues, which can impede a 3'-->5' exoribonuclease in vitro, is able to substitute for the atpB IR in vivo. Strains containing the poly-guanosine tract in place of the atpB 3' IR are phototrophic and accumulate near wild-type levels of discrete atpB transcripts and the ATPase beta-subunit protein. Because these atpB transcripts contain the 18 guanosine residues, and the poly-guanosine tract is not a terminator of transcription, the accumulation of discrete atpB transcripts is likely the result of impediment of 3'-->5' exoribonuclease activity. These findings support a model in which atpB transcripts lacking the 3' IR are degraded by 3'-->5' exoribonuclease activity, and demonstrate that the poly-guanosine tract can be used to study chloroplast RNA metabolism in vivo.