Complex RNA maturation pathway for a chloroplast ribosomal protein operon with an internal tRNA cistron.
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 cotranscription 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 cotranscribed with the ribosomal protein loci and encode stable RNA species of 2.4, 1.8, and 1.4 kilobases.
Christopher
D A
DA
Department of Molecular and Cellular Biology, University of Arizona, Tucson 85721.
Hallick
R B
RB
D000818:Animals; D001483:Base Sequence; D002736:Chloroplasts / Q000378:metabolism*; D005056:Euglena gracilis / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D005810:Multigene Family / Q000235:genetics; D009876:Operon / Q000235:genetics; D012333:RNA, Messenger / Q000378:metabolism*; D012355:RNA, Transfer, Ile / Q000235:genetics*; D012269:Ribosomal Proteins / Q000235:genetics* / Q000378:metabolism
D012333:RNA, Messenger; D012355:RNA, Transfer, Ile; D012269:Ribosomal Proteins