Monoclonal antibodies have been raised against the light-harvesting chlorophyll a/b-binding proteins of photosystem I (LHCI) using a photosystem (PS) I preparation (PSI-200) wild-type from barley (Hordeum vulgare L. cv. Svaløf's Bonus) as the antigen. These antibodies cross-reacted with a minor light-harvesting chlorophyll a/b-protein of PSII (Chla/b-P1=CP29), but not with the major one, LHCII (=Chla/b-P2(**)). Similarly, a monoclonal antibody to Chla/b-P1, elicited by a PSII preparation as the antigen, cross-reacted with LHCI, but not LHCII. This explains why an antigen consisting of LHCII, free of LHCI, but contaminated with Chla/b-P1, can elicit antibodies which cross-react with LHCI. Immunoblot assays showed that LHCI and Chla/b-P1 have at least two epitopes in common. Immunogold labelling of thin-sectioned wild-type thylakoids confirmed a preferential localisation of Chla/b-P1 in grana partition membranes and LHCI in stroma lamellae. The presence of LHCI was demonstrated in barley mutants lacking the PSI reaction centre (viridis-zb (63)) and chlorophyll b (chlorina-f2), and was correlated with the presence of long-wavelength (730 nm) fluorescence emission at 77 K. The mutant viridis-k (23), which has a 77 K long-wavelength fluorescence peak at 720 nm, was shown by immune-blot assay to lack LHCI, although Chla/b-P1 was present.

A cDNA clone encoding the barley photosystem I polypeptide which migrates with an apparent molecular mass of 16 kDa on SDS-polyacrylamide gels has been isolated. The 634 bp sequence of this clone has been determined and contains one large open reading frame coding for a 15 457 Da precursor polypeptide. The molecular mass of the mature polypeptide is 10 821 Da. The amino acid sequence of the transit peptide indicates that the polypeptide is routed towards the stroma side of the thylakoid membrane. The hydropathy plot of the polypeptide shows no membrane-spanning regions. Photosystem I; 16 kDa polypeptide; cDNA sequence; Transit peptide; (Barley)

Monoclonal antibodies have been produced to the two putative 18.3 and 15.2 kD iron-sulphur centre proteins in barley. When photosystem I particles, containing only chlorophyll a-protein 1 and the 18.3 and 15.2 kD proteins, were used as the antigen, thirteen independent clones secreting either IgG or IgM antibodies against the 15.2 kD protein were formed. No hybridomas secreting antibodies to the 18.3 kD protein were detected. When a delipidated polypeptide preparation was the immunogen, we obtained one hybridoma, secreting antibodies of the IgM class to the 18.3 kD protein.These antibodies were used in immune-blot assays. Neither the 18.3 nor 15.2 kD putative iron-sulphur centre proteins were detected in etioplast membranes. However, in the chloroplast membranes of the photosystem I mutant viridis-zb ~' we found small amounts of these proteins.The two proteins were purified by electroelution and then tested by immune-blot assay. Fractions with positive reaction were collected and the amino acid compositions determined. There were significant differences in the amounts ofserine, glycine and histidine between the two proteins.

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