Nucleotide sequence of a region of maize chloroplast DNA containing the 3? end of clpP, exon 1 of rps12 and rpl20 and their cotranscription

Abstract

The chloroplast ribosome of higher plants contains more than 60 proteins [1]. Twenty-one of these proteins are encoded on the plastid genome [2], whereas the genes for the remaining proteins are located in the nuclear DNA [ 1 ]. Most of the ribosomal protein genes on the chloroplast gehome are organized into operons; several of these transcription units also contain genes not encoding components of the translational apparatus, e.g. operons rps2-atplHFA, psaAB-rpsl4 [2, 3 and unpublished transcription results from our lab]. The gene for the chloroplast ribosomal protein L20 (i.e. the chloroplast protein homologous to Escherichia coli L20) forms a cluster with exon 1 of rpsl2 and the gene for the proteolytic subunit (clpP) of a conserved [4] ATP-dependent protease [5]. L20 is one of the proteins essential for the assembly of the large ribosomal subunit in E. coli [6] and it can replace the assembly initiator protein L24 at temperatures below 32 °C [7]. The gene for ribosomal protein $12 is split into three exons in the chloroplast genome of land plants [2]; exons 2 and 3 form a functional operon with rps7 [8], while exon 1 is located more than 20 kb away and is trans-spliced to produce the mature $12 mRNA [e.g. ref. 9]. Here we report the nucleotide sequence of the region from maize chloroplast DNA containing the 3' end ofclpP, exon 1 ofrpsl2 and rpl2Oand evidence from northern blots that the three genes are cotranscribed. The genes are located in the 3.8 kb Barn HI fragment (Barn 10) of maize chloroplast DNA present in the library we have been using [8]. The subcloning, nucleotide sequencing and computer evaluations were done as previously described [10]. The nucleotide sequence is presented in Fig. 1. The 31 amino acids encoded in the 5' region of the sequenced fragment could be identiffed as the C-terminal part of ClpP (Fig. 2). Exon 1 of rpsl2 (the sequence given here corrects and replaces that in ref. 8), encoding the first 38 amino acids of ribosomal protein S 12, is located 133 bp downstream of the termination codon of clpP. Curiously, there is no recognizable ribosome-binding (Shine-Dalgarno) site in front of the ATG initiation codon of this exon. The exon is terminated by the conserved exon-intron bor-