Nonsense mutations intheChlamydomonas chloroplast genethat codesforthelarge subunit ofribulosebisphosphate

ABSTRACT The Chlamydomonas reinhardtii chloroplastmutants18-SBand18-7Glackboththechloroplast-encodedlargesubunit and nuclear-encoded small subunit of the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39). A chloroplast intergenic-suppression model has beenpostulated to account for the genetic instability of 18-5B rever-tants. Here,wehavedeterminedthemolecularbasisofthe18-SBand18-7Gmutants.Theycontain nonsense mutationsclosetothe3' and 5' ends of their large-subunit genes, respectively.Puls-chase experimentsrevealedthatthe18-SBmutantproducesa truncated large subunit that is unstable. In connection withpreviousexperiments, this workidentifies nonsense suppression in the chloroplast. Small subunits are also synthesized butthen degraded in the mutants. Thus, the coordinated absence ofsubunitsis achieved throughdegradation ofthesmallsubunitinthe specific absenceofthelargesubunit.The chloroplast-localized ribulose-1,5-bisphosphate (Rbu-P2) carboxylase/oxygenase (EC 4.1.1.39) holoenzyme