Nucleotide sequence of the clustered genes for the 44 kd chlorophyll a apoprotein and the ?32 kd?-like protein of the photosystem II reaction center in the spinach plastid chromosome

Abstract

A 2,900 base pair DNA segment of the spinach plastid chromosome which encodes the genes for the 44 kd chlorophyll a apoprotein and a “32 kd”-like protein of the photosystem II reaction center has been subjected to sequence and Northern blot analysis. The genes are located almost centrally in the large single-copy segment of the chromosome adjacent to the two genes for the P₇₀₀ chlorophyll a apoproteins of the photosystem I reaction center. The DNA sequence reveals two uninterrupted protein-coding regions of 473 (44 kd chlorophyll a apoprotein) and 353 triplets (“32 kd”-like protein). The latter gene is strikingly similar to the gene for the herbicide-binding “32 kd” protein which maps some 30 kbp distant on the plastid chromosome. The two genes overlap by 50 base pairs but are read in different phases. They may be contranscribed and the RNA modified to give several discrete species ranging in size from 1.6 to 4.6 kb. A presumptive promoter site was only identified for the “32 kd”-like protein, while potential ribosome binding and transcription termination sites are found preceding and following both genes, respectively. The polypeptides possess a high content of hydrophobic amino acids, most of which appear to be clustered in transmembrane spans. The molecular weights of 51,785 (44 kd chlorophyll a apoprotein) and 39,465 (“32 kd”-like protein) derived from the deduced amino acid sequences are higher than the experimentally determined protein sizes. Amino acid codon usage for both genes is highly selective. Comparison of the chlorophyll a apoproteins of spinach reveals regions of sequence homology.