THE PLANT CELL, Vol 2, Issue 12 1181-1190, Copyright © 1990 by American Society of Plant Biologists
Post-Transcriptional Regulation of Organ-Specific Expression of Individual rbcS mRNAs in Lemna gibba
J. Silverthorne and E. M. Tobin
Department of Biology, University of California, Los Angeles, California 90024
Many studies of nuclear genes encoding chloroplast proteins have focused on
the transcriptional regulation of their expression. The genes (rbcS)
encoding the small subunit (SSU) of ribulose-1,5-bisphosphate
carboxylase/oxygenase, a major stromal protein, comprise one such group. We
have examined the role played by post-transcriptional events in determining
the relative levels of individual rbcS mRNAs in different organs of the
aquatic monocot Lemna gibba. L. gibba is unusual among angiosperms in that
its roots are normally exposed to light during growth and contain
chloroplasts. We have found that such roots transcribe rbcS genes and
contain rbcS mRNA. We have used sequence-specific probes from the
3[prime]-untranslated region of six rbcS genes from L. gibba to analyze the
expression of the individual genes in different organs. All six genes were
expressed in steady-state mRNA in fronds grown in constant white light.
However, only five of these were easily detectable in steady-state mRNA
isolated from roots of the same plants, and the relative expression of each
gene varied between the roots and the fronds. In steady-state mRNA, SSU1
was found to be highly expressed in both roots and fronds, whereas SSU40B
was expressed at low levels in the roots as compared with the fronds, and
SSU5B RNA was barely detectable in the roots. The extremely low level of
SSU5B RNA in steady-state root mRNA is likely to be a consequence of
post-transcriptional events because this gene was transcribed at comparable
rates in vitro in nuclei isolated from either roots or fronds. Localization
of individual gene transcripts by in situ hybridization showed that SSU1
and SSU5B are expressed in the same cells in the fronds. Thus, the
mechanism of differential expression is likely to involve an organ-specific
post-transcriptional mechanism.
