The small subunit (SSU) of ribulose 1‐5 bisphosphate carboxylase/oxygenase is a 15 kd protein in Euglena gracilis. The protein is synthesized as a 130 kd precursor as shown by immunoprecipitation of in vitro translation products and confirmed by immunoprecipitation of in vivo pulse‐labeled Euglena proteins. From the published SSU amino acid sequence, an oligonucleotide was synthesized that specifically hybridizes to a large mRNA whose length (approximately 4.3 kb) is consistent with the precursor size. The complete nucleotide sequence of the SSU mRNA was obtained by sequencing a cDNA clone from a lambda gt11 library and completed by direct mRNA sequencing. We report for the first time the complete sequence of a large mRNA and show that it encodes eight consecutive SSU mature molecules. The deduced precursor amino acid sequence shows that the amino terminus of the first SSU molecule is preceded by a 134 amino acid peptide which is cleaved during the maturation process. This long transit peptide exhibits features characteristic of signal peptides involved in the secretion of proteins through the endoplasmic reticulum. This is in agreement with the idea that the third (outer) membrane of the Euglena chloroplast envelope is of endoplasmic reticulum origin.

We have recently shown that, in Euglena gracitis, leader sequences are transferred by trans-splicing to the vast majority of cytoplasmic mRNAs. Trans-splieing is involved in the maturation of the rbcS transcript, which encodes eight small subunits of the ribulose 1,5 bisphosphate carboxylase/ oxygenase. In this report, we show that the Eugh, mt rbcSgene introns are different from introns found in plant rbcS genes, In additiot~ these introns do not have the conserved 5' and 3' border sequences found in introns of euearyotie nuclear-encoded pre.mRNAs, and they do not present any homology with sell:splicing introns of groups I and II. Secondary structure analyses show that the 5' and 3' ends of Eagle/in introns can base-pair, suggesting that an Ulmsaal splicing mechanism exists in Euglena.

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