Plant Molecular Biology 13: 723-725, 1989. © 1989 Kluwer Academic Publishers. Printed in Belgium. Plant Molecular Biology Update Nucleotide sequence of the psbB gene of Euglena gracilis 723 Mario Keller, Jacques Henry Well and C.K. Krishnan Nair 1 Institut de Biologie Mol~culaire des Plantes du CNRS, UniversitOL. Pasteur, 12 rue du General Zimmer, 67084 Strasbourg, France; 1present address: Biochemistry Division, Bhaba Atomic Research Center, Trombay, Bombay 400 085, India Received and accepted 12 July 1989 Key words." Euglena gracilis, chloroplast DNA, psbB gene, introns The psbB gene codes for the '51 kDa' chlorophyl- l a apoprotein of photosystem II. In Euglena gra- cilis, the psbB gene has been shown to be located on restriction fragment Eco H (5.5 kbp) of chloro- plast DNA [ 1]. We present here the nucleotide sequence of the Euglena psbB gene as determined by the dideoxy- nucleotide chain termination method and the de- duced amino acid sequence. As shown in Fig. 1, the Euglena psbB gene is split: 5 exons consisting of 3, 5, 169, 23 and 308 codons are separated by 4 introns of 502, 104, 567 and 572 nucleotides, respectively (the last codon of exons 1 and 3 is split by the intron). The size of the second intron is unusual because up to now small introns were only found in Euglena chloroplast genes coding for components of the transcriptional and trans- lational apparatus [2]. The 5 exons code for a protein of 508 amino acids with a predicted mo- lecular weight of 58.5 kDa. Concerning codon usage, there is a strong bias for A or T in the third position. The amino acid sequence deduced from the nucleotide sequence of the psbB gene is 77.2 ~o homologous with that of its Marchantia polymor- pha counterpart [3]. The 4 introns found in the psbB gene are very A + T-rich (> 81 ~o) with a preference for T over A and present in their 5' and 3' ends structural homologies with the consensus sequences of group II introns [4]. In the 3' pyrim- idine-rich end of these introns, one finds an A potentially involved in the formation of the lariat structure which plays a central role in the splicing mechanism. Nevertheless, these Euglena chloro- plast introns cannot adopt the typical secondary structure of group II introns, structure which brings their 5' and 3' ends in close proximity to permit the splicing. The psbB gene spans a region of 3269 bp and starts 60 bp downstream from the 3' end of the tRNA Gly gene (trnG) [5]. Another 63 bp down- stream from the TAA termination codon of the psbB gene lies the initiation codon of ORF35 (only the first exon and the 5' end of the intron of this split ORF are presented in Fig. 1). The asso- ciation of the psbB gene and ORF35 has only been shown so far in M. polymorpha but not in other plants [3 ]. The nucleotide sequence data reported will appear in the EMBL, GenBank and DDBJ Nucleotide Sequence Databases under the accession number X15903.