Plant Molecular Biology 16: 1085-1087, 1991. © 1991 Kluwer Academic Publishers. Printed in Belgium. Update section Sequence Nucleotide sequence of a cDNA clone encoding the precursor of the 33 kDa protein of the oxygen-evolving complex from wheat J.W. Meadows, 1 A. Hulford, 1, C.A. Raines 2 and C. Robinson 1. 1Department of Biological Sciences, University of Warwick, Coventry, CV4 7AL, UK (*author for correspondence); z Department of Biology, University of Essex, Colchester, UK Received 19 February 1991; accepted 28 February 1991 1085 Key words: chloroplasts, oxygen evolution, precursor proteins, thylakoid membranes, wheat cDNA clone The light-driven oxidation of water in the thy- lakoid membrane is carried out by photosystem II, a multisubunit complex containing both integral and extrinsic components. The water- oxidation reaction takes place on the lumenal side of the membrane and is catalysed by a group of proteins, often termed the oxygen-evolving com- plex, of which three proteins of 33, 23 and 16 kDa are particularly prominent [2, 5]. The 33 kDa protein is believed to stabilise an essential manganese centre, whereas the 23 kDa and 16kDa proteins probably assume regulatory functions [ 1, 6]. All three proteins are synthesised in the cytosol as larger precursors containing ami- noterminal pre-sequences [11] and import into the thylakoid lumen is believed to take place by a two-step mechanism. Precursor proteins are ini- tially transported into the stroma and cleaved to intermediate forms by a stromal processing pep- tidase, after which these intermediates are trans- ferred across the thylakoid membrane and cleaved to the mature sizes by a second, thylakoid proces sing peptidase [ 3 ]. We have determined the N-terminal amino acid sequence of the wheat 33 kDa protein, and have used specific antisera to isolate a full-length cDNA encoding the precursor protein from a wheat leaf library in 2gtll. The nucleotide and deduced amino acid sequences of this cDNA, p33K-2, are shown in Fig. 1. Amino acid residues 80-102 precisely match the sequence determined by N-terminal Edman degradation of the purified protein; the precursor protein therefore consists of a mature protein (246 residues) preceded by a pre-sequence of 79 amino acids. The mature wheat protein shows a high degree of conser- vation with the corresponding pea [9, 10] and spinach [7] proteins, with 82~o and 81~o of the residues being identical, respectively. The pre-sequence of the wheat 33 kDa protein is shorter than those of the corresponding spinach and pea proteins, by 5 and 2 residues, respec- tively. The intermediate cleavage site has yet to be determined, but it is believed that the envelope transfer and thylakoid transfer domains are of approximately equal length [3]. A comparison of the wheat and spinach pre-sequences is shown in Fig. 2; overall, these sequences exhibit consider- ably less homology (54 ~o residue identity) than do the mature sequences, but it is notable that the second, thylakoid transfer domains are more highly conserved than the envelope transfer domains. Only the amino-terminal sections of the envelope transfer domains are highly conserved, although it may be significant that the positions of basic residues are also conserved. The thylakoid The nucleotide sequence data reported will appear in the EMBL GenBank and DDBJ Nucleotide Sequence Databases under the accession number X57408.