Institute of Botany, Academia Sinica, Taipei, Taiwan, Rep. of China
We have studied chloroplast psa A-psa B-rps 14 gene cluster from the Japonica cultivar of Oryza sativa (cv. Tainong 67) in the past few years (Chen et al, 1988, 1990). The presence of sequence heterogeneity in the gene cluster is observed in the overlapping clones isolated from the chloroplast DNA library in lambda gtll. In
Fig. 1. Comparison of nucleotide sequences of psaA gene and upstream IRF 170
exon 3 region between rice cultivars Tainong 67 (upper sequence) and
Nihonbare (lower sequence). Only bases which differ at the equivalent
position are shown for Nihonbare. Underlined sequence is termination codon
for IRF 170 exon 3.
**** indicates the identical sequences of the two cultivars from -1 to -600.
late 1989 we completed sequencing one of the overlapping clones which covered the psaA gene (2253 bp) encoding one of the large subunits of chlorophyll a- protein complex in photosystem I and the upstream 810 bp- region. Using the molecular biology software package PC/GENE (Moore et al. 1988) to analyze the psaA sequence, the gene product is predicted to be 750 amino acids in length and have a calculated molecular weight of 83,237 with pI value of 6.69. Thirteen membrane associated alpha-helices are predicted in the highly hydophobic protein by the program of Rao and Argos (1986).
At the same time, Hiratsuka et al (1989) reported the complete DNA sequence of the rice chloroplast genome of another Japonica variety (cv. Nihonbare). We therefore make comparison between the sequences of the two closely related cultivars (Fig. 1). In the coding region of psaA gene, there are 7 nucleotides changes which are all located at the N-terminal half region. The sequences in the C-terminal half region are identical in both cultivars. Only 2 nucleotide changes in IRF 170 exon 3 are observed in the upstream 810 bp-region between the two cultivars. Fig. 1 thus shows only the psaA sequences of the N-terminal half region together with the upstream region containing IRF 170 exon 3. All the nucleotide changes are nonsynonomous leading to five amino acid changes in psaA and one amino acid change in IRF 170 exon 3.
When compared to the maize sequence (Fish et al. 1985) at the 7 equivalent positions of psaA and the two positions in IRF 170 exon 3 at which nucleotide substitutions occur in two rice cultivars, all the 9 equivalent nucleotides are fully conserved between Tainong 67 and maize. Furthermore, compared to the dicot spinach sequence (Kirsh et al. 1986), all the five corresponding amino acids in psaA are conserved between Tainong 67 and spinach. These comparisons suggest the observed sequence differences between the two rice cultivars is very likely due to error in sequencing of cv. Nihonbare, rather than due to true sequence differences.
This work was supported by Grant NSC 79-0211-B001-41 from National Science Council, Rep. of China.
References
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