Nucleotide sequence of the frxC, petB and trnL genes in the chloroplast genome of Chlamydomonas reinhardtii.

Abstract

The nucleotide sequence of a 4262 bp DNA fragment was determined from the chloroplast genome of the green alga Chlamydomonas reinhardtii (wild-type, CC125, see [1]). Sequence analysis revealed two protein-encoding genes (frxC and petB) and one tRNA gene (trnL) (Fig. 1). In addition, the intergenic sequences predict three stable stem-loop structures possibly involved in regulating the accumulation of the gene transcripts. This DNA fragment is located upstream of a ribosomal protein gene cluster starting with the rpl23 gene (unpublished data), and is within the 7.6 kb Eco9 DNA fragment [1] which is equivalent to the R20 DNA fragment named by Rochaix [2]. The only frxC gene reported previously has been found in the chloroplast genome of the liverwort Marchantia polymorpha [4]. The putative protein product predicted from this gene shows significant sequence similarity to the ntf/-/ gene product of nitrogen-fixing bacteria, and appears to be an iron (Fe)-binding protein with conserved cysteine residues. It was not certain whether the frxC gene is a functional and typical chloroplast gene, because chloroplasts are not known to possess NifH activities, and because the frxC gene is not present in chloroplast genomes of higher plants examined [ e.g. 5 ]. Our finding of the frxC gene in the chloroplast genome of a green alga (Fig. 1), together with the previous finding of this gene in liverwort [4], suggests that the frxC gene is a common chloroplast gene which has been retained in chloroplasts of green algae and lower plants but transferred to the nucleus in higher plants. Amino acid sequence comparisons among the two chloroplast FrxC proteins and the three types of NifH proteins [6] revealed extensive sequence similarities (Fig. 2). The FrxC protein sequence of C. reinhardtii is 88~o identical to that of liverwort. This high degree of structural conservation, comparable to that of many photosynthetic complex proteins, suggests conserved cellular functions of the frxC genes. Sequence identities between the C. reinhardtii FrxC protein and the NifH proteins range from 32~o to 35~o, suggesting that these proteins are evolutionarily related. The FrxC sequence is about equally similar to each of the three types of NifH proteins, suggesting that the frxC gene diverged from an ancestral mf/-/gene that subsequently gave rise to the three contemporary types of n/fH genes [6]. Amino