THE PLANT CELL, Vol 2, Issue 7 659-671, Copyright © 1990 by American Society of Plant Biologists
Complex RNA Maturation Pathway for a Chloroplast Ribosomal Protein Operon with an Internal tRNA Cistron
D. A. Christopher and R. B. Hallick
Department of Molecular and Cellular Biology, University of Arizona, Biosciences West, Room 524, Tucson, Arizona 85721
We have studied the expression of a large chloroplast ribosomal protein
operon from Euglena gracilis that resembles the Escherichia coli S10 and
spc ribosomal protein operons. We present evidence that 11 ribosomal
protein genes, a tRNA gene, and a new locus, orf214/orf302, are expressed
as a single transcription unit. The primary transcript also contains at
least 15 group II and group III introns. Gene-specific probes for each
ribosomal protein gene, orf214/orf302, and for trnl hybridized to a common
pre-mRNA of an estimated size of 8.3 kilobases. This is the RNA size
predicted for a full-length transcript of the entire operon after splicing
of all 15 introns. Polycistronic ribosomal protein mRNAs accumulated
primarily as spliced hepta-, hexa-, penta-, tetra-, tri-, and dicistronic
mRNAs, which were presumed to arise by stepwise processing of the
8.3-kilobase pre-mRNA. A novel finding was the co-transcription of the trnl
gene as an internal cistron within the ribosomal protein operon. Several
combined mRNA/tRNA molecules, such as the pentacistronic
rpl5-rps8-rpl36-trnl-rps14, were characterized. The occurrence of the
orf214/orf302 is a unique feature of the Euglena operon, distinguishing it
from all chloroplast and prokaryotic ribosomal protein operons
characterized to date. The orf214/orf302 are not similar to any known genes
but are co-transcribed with the ribosomal protein loci and encode stable
RNA species of 2.4, 1.8, and 1.4 kilobases.
