A CHLOROPLAST TRANSCRIPT LACKING THE 3' INVERTED REPEAT IS DEGRADED BY 3'-]5' EXORIBONUCLEASE ACTIVITY

Chlamydomonas reinhardtii strains harboring deletions of the chloropla st 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 i mpede a 3' --> 5' exoribonuclease in vitro, is able to substitute for the atpB IR in vivo. Strains containing the poly-guanosine tract in pl ace of the atpB 3' IR are phototrophic and accumulate near wild-type l evels of discrete atpB transcripts and the ATPase beta-subunit protein . Because these atpS transcripts contain the 18 guanosine residues, an d the poly-guanosine tract is not a terminator of transcription, the a ccumulation of discrete atpB transcripts is likely the result of imped iment 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-guanosin e tract can be used to study chloroplast RNA metabolism in vivo.