The Synthesis and Assembly of Functional High and Low Light LH2 Antenna Complexes from Rhodopseudomonas palustris in Rhodobacter sphaeroides

Gregory J.S Fowler,C Neil Hunter

doi:10.1074/jbc.271.23.13356

Abstract

Photosynthetic bacteria respond to lowered light intensity by increasing the level of the peripheral light-harvesting (LH2) complex. Several species possess an additional mechanism, responding to variations in light conditions by making different types of LH2 complex. However, the study of these complexes in isolation and in the native membrane environment has not been possible. Therefore two LH2 gene pairs from Rhodopseudomonas palustris, associated, respectively, with high light (pucBAa) and low light (pucBAd) growth conditions, were expressed in Rhodobacter sphaeroides. The high light LH2 complex PucBAa was synthesized at appreciable levels in R. sphaeroides, had near-infrared absorption bands at approximately 800-855 nm, and was able to transfer energy efficiently to the native LH1 complex. In contrast, the low light complex PucBAd was found at comparatively low levels, had absorption bands at approximately 797-830 nm, and did not transfer energy to the native LH1 complex efficiently. These observations are discussed in the light of site-directed studies on the R. sphaeroides LH2 complex, and the recently elucidated Rhodopseudomonas acidophila 10050 LH2 structure. Potentially important residues for energy transfer between LH2 and LH1 complexes are identified, as well as some of the factors that influence stability and assembly of LH2 complexes, such as the N-terminal sequences of their protein subunits and their carotenoid binding sites.

Highlights

In photosynthesis, light energy is absorbed by carotenoid andchlorophyll molecules, most of which are found in the light-harvesting (LH)1 complexes, and is transferred via the different pigments to the photochemical reaction center (RC) with high efficiency (see, for example, van Grondelle et al (1994))
The two LH2 complexes chosen for the present study, encoded by the genes pucBAa and pucBAd, are described in Tadros and Waterkamp (1989) and Tadros et al (1993) for Rps. palustris strain 1e5; the protein sequences of these complexes (PucBAa and PucBAd) match those given in the Ph.D. thesis of Evans (1989) for Rps. palustris strain French, but with a different nomenclature
In this latter work PucBAa is named ␣1, and PucBAd is named ␣2. (Protein sequences for the LH2 complexes for Rps. palustris strain French are given in Brunisholz and Zuber (1992).)

Summary

Introduction

Light energy is absorbed by carotenoid and (bacterio)chlorophyll molecules, most of which are found in the light-harvesting (LH)1 complexes, and is transferred via the different pigments to the photochemical reaction center (RC) with high efficiency (see, for example, van Grondelle et al (1994)). In order to measure the ability of the 800-nm absorbing pigments to transfer energy to the B850 pigments within the heterologously synthesized Rps. palustris PucBAa and PucBAd LH2 complexes, excitation spectra in the near infrared region of the spectrum were measured (Fig. 3).

Results

Conclusion