Assembly and composition of the chlorophyll a-b light-harvesting complex of barley (Hordeum vulgare L.): Immunochemical analysis of chlorophyll b-less and chlorophyll b-deficient mutants

Abstract

The chlorina-f2 mutant of barley (Hordeum vulgare L.) contains no chlorophyll b in its light-harvesting antenna, whereas the chlorina-103 mutant contains approximately 10% of the chlorophyll b found in wild-type. The absolute chlorophyll antenna size for Photosystem-II in wild-type, chlorina-103 and chlorina-f2 mutant was 250, 58 and 50 chlorophyll molecules, respectively. The absolute chlorophyll antenna size for Photosystem-I in wild-type, chlorina-103 and chlorina-f2 mutant was 210, 137 and 150 chlorophyll molecules, respoectively. In spite of the smaller PS I antenna size in the chlorina mutants, immunochemical analysis showed the presence of polypeptide components of the LHC-I auxiliary antenna with molecular masses of 25, 19.5 and 19 kDa. The chlorophyll a-b-binding LHC-II auxiliary antenna of PS II contained five polypeptide subunits in wild-type barley, termed a, b, c, d and e, with molecular masses of 30, 28, 27, 24 and 21 kDa, respectively. The polypeptide composition of the LHC-II auxiliary antenna of PS II was found to be identical in the two mutants, with only the 24 kDa subunit d present at an equal copy number per PS II in each of the mutants and in the wild-type barley. This d subunit assembles stably in the thylakoid membrane even in the absence of chlorophyll b and exhibits flexibility in its complement of bound chlorophylls. We suggest that polypeptide subunit d binds most of the chlorophyll associated with the residual PS II antenna in the chlorina mutants and that is proximal to the PS II-core complex.

Chlorophyll a/b-Binding Proteins, Pigment Conversions, and Early Light-Induced Proteins in a Chlorophyll b-less Barley Mutant

“…Chlorina f2 contained, on a protein basis, slightly less PSI but somewhat more PSII then the wild type, consistent with the findings of Harrison et al (1993). In ImL plants, the PsaD and PsaL antibodies gave conflicting results.…”

“…We also wanted to relate the pigment content to the number of PSI and PSII centers. It is known that wild-type barley contains 1.7 PSI and 3 PSII centers/1000 Chl, whereas the corresponding figures for cklorina f.2 are about 3 and 10 (Harrison et al, 1993). In the ImL plants, the antenna size of PSI is probably about 100 Chl a and of PSII approximately 50 Chl a, the sizes of the core antenna.…”

“…The barley (Hordeum vulgare L.) chlorina f2 mutant (Simpson et al, 1985) is the most wellcharacterized Chl b-deficient mutant. Severa1 studies have addressed its LHC polypeptide composition Green, 1987, 1988;Morrissey et al, 1989;Harrison and Melis, 1992;Harrison et al, 1993), but it is still not fully disclosed. A major obstacle for correct identification and quantification lies in the immunological cross-reactivity generally observed between different LHC proteins.…”

Pigment Binding of Photosystem I Light-harvesting Proteins

“…With regard to LHC biogenesis the availability of single pigment species is of differential importance for the various LHCs as revealed by altered stoichiometries of the LHCs in the thylakoid membrane as a consequence of different light treatments (22, 61, 64 -66) or disruption of pigment synthesis by mutations (23,60,67,68). Our experiments show that the absence of the ␤,␤-carotenoids vio and ␤-car did not impair LHCI formation/stability.…”

“…This finding may be of interest with regard to the in vivo situation because it indicates that the pigment composition of LHCIs may depend not only on the protein structure but also on the pigments available during LHCI formation. This was proposed for Chl binding by LHCI in barley (60), and results obtained with maize seedlings exposed to intermittent light also revealed flexibility in pigment binding in vivo (61). Additionally, reconstitution studies demonstrated the interchangeability of LHC proteins and carotenoids from alga and higher plants (21,62,63).…”