The Rhodobacter sphaeroides PufX protein is not required for photosynthetic competence in the absence of a light harvesting system

Abstract

The effects of deletion of the gene encoding the PufX protein from Rhodobacter sphaeroides have been examined using bacterial strains with simplified photosystems. We find that the PufX protein is required for photosynthetic growth in strains which have the LHI antenna complex, but is not required in a reaction centre-only strain, suggesting that the PufX protein does not directly facilitate cyclic electron transfer between the reaction centre and the cytochrome bc, complex. The influence of PufX and carotenoid type on the size of the reaction center/LHl core complex has also been examined in these strains.

Mutation of the Ser2 codon of the light-harvesting B870 alpha polypeptide of Rhodobacter capsulatus partially suppresses the pufX phenotype

“…This category also includes the strain described by McGlynn et al, which completely lacked the B870 light-harvesting complex as well as the PufX protein and was capable of phototrophic growth equal to that of the isogenic pufX ϩ strain (22).…”

“…We use the information summarized above and the results of a number of independent studies (2,7,17,18,19,21,22) to propose a speculative model in which the PufX protein creates a void in the structure of the B870 antenna complex, where a local absence of pigments allows passage of quinones between the reaction center and the b-c 1 complex.…”

“…The B870 Bchl content of R. sphaeroides pufX ϩ and pufX deletion strains has been carefully measured and found to increase in pufX deletion strains by 1.16 to 1.49 B870 per reaction center (22). These values are what would result from the displacement by PufX of pigment from one to four ␣/␤ dimers per B870 complex.…”

Membrane biogenesis in anoxygenic photosynthetic prokaryotes

“…sphaeroides . The finding was further supported by the localization of second-site suppressor mutants in the pufBA genes encoding the LH 1-α-and β-apoproteins (Lilburn et al 1995), and the lack of a PufX requirement in the absence of LH 1 as shown in Neil Hunter's laboratory (McGlynn et al 1994). These findings have suggested that in the absence of PufX, LH 1 tightly surrounds the reaction center and impedes UQ/UQH 2 movements and that the suppression of this defect results from a destabilization of LH 1, enhancing UQ exchange.…”

“…capsulatus, as demonstrated by the Beatty group (Lilburn et al 1992). The latter workers proposed that PufX functions in promoting UQ/UQH 2 exchange between sites (Jungas et al 1999), these models are based upon findings in pufX deletion strains which include a 1.2-1.5-fold increases in the LH 1-reaction-center ratios (McGlynn et al 1994), and the observation of a monomeric LH 1-reaction-center complex in sucrose gradients (Francia et al 1999). …”