Dynamics of energy transfer from lycopene to bacteriochlorophyll in genetically-modified LH2 complexes of Rhodobacter sphaeroides.

LH2 complexes from Rb. sphaeroides were modified genetically so that lycopene, with 11 saturated double bonds, replaced the native carotenoids which contain 10 saturated double bonds. Tuning the S1 level of the carotenoid in LH2 in this way affected the dynamics of energy transfer within LH2, which were investigated using both steady-state and time-resolved techniques. The S1 energy of lycopene in n-hexane was determined to be approximately 12 500 +/- 150 cm(-1), by direct measurement of the S1-S2 transient absorption spectrum using a femtosecond IR-probing technique, thus placing an upper limit on the S1 energy of lycopene in the LH2 complex. Fluorescence emission and excitation spectra demonstrated that energy can be transferred from lycopene to the bacteriochlorophyll molecules within this LH2 complex. The energy-transfer dynamics within the mutant complex were compared to wild-type LH2 from Rb. sphaeroides containing the carotenoid spheroidene and from Rs. molischianum, in which lycopene is the native carotenoid. The results show that the overall efficiency for Crt --> B850 energy transfer is approximately 80% in lyco-LH2 and approximately 95% in WT-LH2 of Rb. sphaeroides. The difference in overall Crt --> BChl transfer efficiency of lyco-LH2 and WT-LH2 mainly relates to the low efficiency of the Crt S(1) --> BChl pathway for complexes containing lycopene, which was 20% in lyco-LH2. These results show that in an LH2 complex where the Crt S1 energy is sufficiently high to provide efficient spectral overlap with both B800 and B850 Q(y) states, energy transfer via the Crt S1 state occurs to both pigments. However, the introduction of lycopene into the Rb. sphaeroides LH2 complex lowers the S1 level of the carotenoid sufficiently to prevent efficient transfer of energy to the B800 Q(y) state, leaving only the Crt S1 --> B850 channel, strongly suggesting that Crt S1 --> BChl energy transfer is controlled by the relative Crt S1 and BChl Q(y) energies.

The Vpr protein of human immunodeficiency virus type 1 binds to nucleocapsid protein p7 in vitro.

The Vpr protein of human immunodeficiency virus type 1 (HIV-1) is incorporated into the virion by the Gag polyprotein precursor Pr55gag. The importance of the p6gag sequence at the C-terminal end of Pr55gag has a crucial role in Vpr incorporation. To identify the Gag sequences directly involved in Vpr binding, we compared the Vpr binding affinities of the 71 amino acid nucleocapsid protein p7, the C-terminal peptide (35-71) p7C and p6gag by affinity chromatography. p7 and p7C have the strongest Vpr binding activities compared to p6gag. These results suggest that the nucleocapsid protein and its C-terminal domain may be important for the incorporation of Vpr into the mature HIV-1 virion and the subsequent localisation of viral nucleic acid to the cell nucleus by Vpr.