Characterisation of the LH2 spectral variants produced by the photosynthetic purple sulphur bacterium Allochromatium vinosum

Abstract

This study systematically investigated the different types of LH2 produced by Allochromatium (Alc.) vinosum, a photosynthetic purple sulphur bacterium, in response to variations in growth conditions. Three different spectral forms of LH2 were isolated and purified, the B800-820, B800-840 and B800-850 LH2 types, all of which exhibit an unusual split 800 peak in their low temperature absorption spectra. However, it is likely that more forms are also present. Relatively more B800-820 and B800-840 are produced under low light conditions, while relatively more B800-850 is produced under high light conditions. Polypeptide compositions of the three different LH2 types were determined by a combination of HPLC and TOF/MS. The B800-820, B800-840 and B800-850 LH2 types all have a heterogeneous polypeptide composition, containing multiple types of both α and β polypeptides, and differ in their precise polypeptide composition. They all have a mixed carotenoid composition, containing carotenoids of the spirilloxanthin series. In all cases the most abundant carotenoid is rhodopin; however, there is a shift towards carotenoids with a higher conjugation number in LH2 complexes produced under low light conditions. CD spectroscopy, together with the polypeptide analysis, demonstrates that these Alc. vinosum LH2 complexes are more closely related to the LH2 complex from Phs. molischianum than they are to the LH2 complexes from Rps. acidophila.

Adaptation of Rhodopseudomonas acidophila strain 7050 to growth at different light intensities: what are the benefits to changing the type of LH2?

“…15 The most discernible low-light LH2 complex from Rhodopseudomonas acidophila 7050 (purple), the subject of this paper, exhibits a blue-shifted B850 peak at a maximum of 824 nm. 16 For this reason, the complex is referred to as a B800-820 LH2 complex and the old nomenclature of LH3 is no longer used.…”

Spectral heterogeneity and carotenoid-to-bacteriochlorophyll energy transfer in LH2 light-harvesting complexes from Allochromatium vinosum

“…Several investigations of the spectroscopic properties of LH2 complexes from purple photosynthetic bacteria have shown that, despite major shifts in the position of the B850 BChl a Q Y and carotenoid absorption bands induced by alterations in bacterial growth conditions, the symmetry of the pigment-protein complex, and hence the exciton structure that produced the NIR BChl a absorption bands in the first place, remained unaltered Carey et al 2014). This is particularly evident in studies on Rbl.…”

“…Allochromatium vinosum is quite different in this respect in that it has been reported to have multiple puc BA genes (Weissgerber et al 2011) that are capable of producing many different types of a and b apoproteins that may assemble within a single LH2 ring and lead to both structural and spectral heterogeneity (Zuber and Brunisholz 1993;Kennis et al 1997;Carey et al 2014). The present work seeks to examine the effect of this heterogeneity on the spectral and energy transfer properties of the carotenoids and BChl a pigments bound in three LH2 complexes, B800-820, B800-840, and B800-850 produced when Alc.…”

“…vinosum (formerly Chromatium (Ch.) vinosum strain D) is a photosynthetic purple sulfur bacterium that possesses the remarkable ability to alter the absorption spectral characteristics of its LH2 complex depending on the environmental conditions under which the organism is grown (Wassink et al 1939;Garcia et al 1966;Hayashi and Morita 1980;Carey et al 2014). With variations in light intensity, temperature or type of reduced sulfur nutrient in the growth media, the position of the 850 nm absorption band can be blue-shifted to either 840 or 820 nm.…”

Functional characteristics of spirilloxanthin and keto-bearing Analogues in light-harvesting LH2 complexes from Rhodobacter sphaeroides with a genetically modified carotenoid synthesis pathway

“…sphaeroides crtI::crtI PA are of particular interest among the entire set of engineered strains [17] because, aside from LH2 complexes from purple sulfur bacteria that have small quantities of carotenoids with N = 12 or 13 [15,18,19], no other LH2 complexes are known to preferentially bind (keto-)carotenoids with such long conjugation lengths (N ≥ 13). To assess the impact of high N value carotenoids on the functional properties of these LH2 complexes, the carotenoid electronic, vibrational, and excited-state properties as well as Car → BChl energy transfer were examined using static and ultrafast optical techniques (at room temperature and 77 K) and RR spectroscopy.…”