Diffusion of Phycobilisomes on the Thylakoid Membranes of the Cyanobacterium Synechococcus 7942

Abstract

A variant of fluorescence recovery after photobleaching allows us to observe the diffusion of photosynthetic complexes in cyanobacterial thylakoid membranes in vivo. The unicellular cyanobacterium Synechococcus sp. PCC7942 is a wonderful model organism for fluorescence recovery after photobleaching, because it has a favorable membrane geometry and is well characterized and transformable. In Synechococcus 7942 (as in other cyanobacteria) we find that photosystem II is immobile, but phycobilisomes diffuse rapidly on the membrane surface. The diffusion coefficient is 3 ؋ 10 ؊10 cm 2 s ؊1 at 30°C. This shows that the association of phycobilisomes with reaction centers is dynamic; there are no stable phycobilisome-reaction center complexes in vivo. We report the effects of mutations that change the phycobilisome size and membrane lipid composition. 1) In a mutant with no phycobilisome rods, the phycobilisomes remain mobile with a slightly faster diffusion coefficient. This confirms that the diffusion we observe is of intact phycobilisomes rather than detached rod elements. The faster diffusion coefficient in the mutant indicates that the rate of diffusion is partly determined by the phycobilisome size. 2) The temperature dependence of the phycobilisome diffusion coefficient indicates that the phycobilisomes have no integral membrane domain. It is likely that association with the membrane is mediated by multiple weak interactions with lipid head groups. 3) Changing the lipid composition of the thylakoid membrane has a dramatic effect on phycobilisome mobility. The results cannot be explained in terms of changes in the fluidity of the membrane; they suggest that lipids play a role in controlling phycobilisome-reaction center interaction.

Phycobilisome Diffusion Is Required for Light-State Transitions in Cyanobacteria

“…A 633-nm red He-Ne laser was used to selectively excite phycocyanin, and fluorescence emission was monitored at .665 nm. At room temperature, most fluorescence at these wavelengths comes from the phycobilisome cores, so these settings can be used to monitor the diffusion of intact phycobilisomes (Sarcina et al, 2001). Prior to the measurement, cells were grown in the presence of 0.5% dimethylsulfoxide; this generates elongated cells that are more suitable for quantitative FRAP measurements (Aspinwall et al, 2004).…”

“…We therefore carried out FRAP measurements using a laser-scanning confocal microscope as described previously (Sarcina et al, 2001;Aspinwall et al, 2004). A 633-nm red He-Ne laser was used to selectively excite phycocyanin, and fluorescence emission was monitored at .665 nm.…”

“…FRAP measurements show that the phycobilisomes are highly mobile, with diffusion coefficients typically around 4 3 10 210 cm 2 s 21 (Sarcina et al, 2001;Aspinwall et al, 2004). By contrast, PSII appears completely immobile (Mullineaux et al, 1997;Sarcina et al, 2001). This indicates that there is no stable association between phycobilisomes and PSII, and it is likely that the association of phycobilisomes with PSI is also transient.…”

“…This indicates that there is no stable association between phycobilisomes and PSII, and it is likely that the association of phycobilisomes with PSI is also transient. Studies with mutant strains of Synechococcus 7942 have shown that the phycobilisome diffusion coefficient is influenced by a number of factors including phycobilisome size and membrane lipid composition (Sarcina et al, 2001) and the oligomerization of PSI (Aspinwall et al, 2004). The physiological role of phycobilisome mobility has remained unclear.…”

“…The cyanobacterium Synechococcus sp. PCC7942 (Synechococcus 7942) has relatively large cells and a very regular thylakoid membrane organization, allowing quantitative measurements of the diffusion coefficients of membrane components (Sarcina et al, 2001;Mullineaux and Sarcina, 2002). FRAP measurements show that the phycobilisomes are highly mobile, with diffusion coefficients typically around 4 3 10 210 cm 2 s 21 (Sarcina et al, 2001;Aspinwall et al, 2004).…”