Projection structures of three photosynthetic complexes from Rhodobacter sphaeroides: LH2 at 6 A, LH1 and RC-LH1 at...

Area of Science:

• Bacteriology
• Structural Biology
• Biochemistry

Background:

• Photosynthetic bacteria utilize light-harvesting complexes for efficient light capture.
• Understanding the structure of these complexes is crucial for elucidating energy transfer mechanisms.

Purpose of the Study:

• To determine the high-resolution structure of photosynthetic complexes from Rhodobacter sphaeroides.
• To investigate the arrangement and subunit composition of light-harvesting complex 2 (LH2), light-harvesting complex 1 (LH1), and the reaction centre-light-harvesting complex 1 photounit (RC-LH1).

Main Methods:

• Purification of LH2, LH1, and RC-LH1 complexes from Rhodobacter sphaeroides.
• Reconstitution of purified complexes into two-dimensional (2-D) crystals.
• Negative stain electron microscopy and image processing to generate projection maps at 25 Å and 6 Å resolution.

Main Results:

• LH1 rings were found to be composed of approximately 15–17 alphabeta subunits with a diameter similar to Rhodospirillum rubrum.
• The reaction centre (RC) was localized within the LH1 ring in RC-LH1 photounits, with evidence suggesting a non-unique orientation.
• LH2 complexes formed ordered tubular crystals, yielding a projection map showing a ring of nine alphabeta subunits with a tilted symmetry axis.

Conclusions:

• The study provides detailed structural insights into key photosynthetic complexes of Rhodobacter sphaeroides.
• The findings contribute to understanding the organization and assembly of bacterial photosynthetic machinery.
• The structural data supports models of light energy capture and transfer in purple bacteria.