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Contradictory results on the effectiveness of energy transfer from the light harvesting complex 2 (LH2) directly to the reaction center (RC) in mutant strains lacking the core light-harvesting complex 1 (LH1) have been obtained with cells of Rhodobacter capsulatus and Rhodobacter sphaeroides. A LH1(-) mutant of Rhodovulum sulfidophilum, named rsLRI, was constructed by deletion of the pufBA genes, resulting in a kanamycin resistant photosynthetically positive clone. To restore the wild type phenotype, a complemented strain C2 was constructed by inserting in trans a DNA segment containing the pufBA genes. Light-induced FTIR difference spectra indicate that the RC in the rsLRI mutant and in the C2 complemented strains are functionally and structurally identical with those in the wild type strain, demonstrating that the assembly and the function of the RC is not impaired by the LH1 deletion. The photosynthetic growth rate of the rsLRI strain increased with decreasing light intensity. At 50 W m(-2 )no photosynthetic growth was observed. These results indicate that the light energy harvested by the LH2 complex was not or inefficiently transferred to the RC; thus most of the energy necessary for photosynthetic growth is in the LH1(-) strain directly absorbed by the RC. It is supposed that in the mutant strain, RC and LH2 cannot interact in an efficient way.

The primary electron donor P700 of photosystem I is a dimer comprised of chlorophyll a (P(B)) and chlorophyll a' (P(A)). P(A) is involved in a hydrogen bond network with several surrounding amino acid residues and a nearby water molecule. To investigate the influence of hydrogen bond interactions on the properties of P700, the threonine at position A739, which donates a putative hydrogen bond to the 13(1)-keto group of P(A), was replaced with valine, histidine, and tyrosine in Chlamydomonas reinhardtii using site-directed mutagenesis. Growth of the mutants was not impaired. (i) The (P700(+)* - P700) FTIR difference spectra of the mutants lack a negative band at 1634 cm(-1) observed in the wild-type spectrum and instead exhibit a new negative band between 1658 and 1672 cm(-1) depending on the mutation. This band can therefore be assigned to the 13(1)-keto group of P(A) which is upshifted to higher frequencies upon removal of the hydrogen bond. (ii) The main bleaching band in the Q(y)() region of the (P700(+)* - P700) and ((3)P700 - P700) absorption difference spectra is blue shifted for the mutants by approximately 6 nm compared to that of the wild type. A blue shift is also observed for the main bleaching in the Soret region. (iii) The (P700(+)* - P700) CD difference spectrum of the wild type reveals two bands at 694 nm (positive CD) and 680 nm (negative CD) of approximately equal area. For each mutant, these two components are blue-shifted to the same extent. The results strongly suggest that a blue shift of the Q(y) absorption band of P(A) is responsible for a blue shift of the exciton bands. (iv) Redox titrations yielded a decrease in the midpoint potential for the oxidation of P700 by 32 mV for the exchange of Thr against Val. (v) ENDOR spectroscopy shows that the hfc of the methyl protons at position 12 of the spin-carrying Chl P(B) is decreased due to the removal of the hydrogen bond to P(A). This indicates a redistribution of spin density in P700(+)* compared to that in the wild type. This gives evidence for an electronic coupling between the two halves of the dimer in the wild type and mutants.