THE PLANT CELL, Vol 5, Issue 4 389-402, Copyright © 1993 by American Society of Plant Biologists
Nuclear Mutants of Maize with Defects in Chloroplast Polysome Assembly Have Altered Chloroplast RNA Metabolism
A. Barkan
Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403-1229
The molecular basis for the photosynthetic defect in four nuclear mutants
of maize was investigated. Mutants hcf7, cps1-1, cps1-2, and cps2 contained
reduced levels of many chloroplast-encoded proteins without corresponding
deficiencies in chloroplast mRNAs. Many chloroplast mRNAs were associated
with abnormally few ribosomes, indicating that the protein deficiencies
were due to global defects in chloroplast translation. These mutants were
used to study the effects of reduced ribosome association on the metabolism
of chloroplast RNAs. The level of the rbcL mRNA was reduced fourfold in
each mutant, but was unaltered in other nonphotosynthetic mutants with
normal chloroplast translation. These results suggest that the rbcL mRNA is
destabilized as a consequence of its decreased association with ribosomes.
The fact that many other chloroplast mRNAs accumulated to normal levels
demonstrated that a decreased association with ribosomes does not
significantly alter their stabilities or processing. hcf7 seedlings had a
gross defect in the processing of the 16S rRNA: the primary lesion in this
mutant may be a defect in 16S rRNA processing itself.
