Friday Feature: Carboxysome spacing

I promise that not every Friday Feature will be a movie, but since I mentioned the Silver lab’s ambitions to control how cells use light energy in the last post, here is a lovely movie of cyanobacteria growing and dividing, in which you can see the remarkably regular spacing of labeled carboxysomes.  Carboxysomes are where much of the magic of carbon fixation happens: they’re said to be responsible for about 40% of all the carbon fixation on the planet.  Dave Savage got interested in how they work, and, not unnaturally, wanted to see what they look like.  In this movie they are labeled with YFP.

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Savage DF, Afonso B, Chen AH, Silver PA.  2010. Spatially ordered dynamics of the bacterial carbon fixation machinery. Science.  327 1258-61. PMID: 20203050

The remarkable regularity you see is not just an illusion: carboxysomes are evenly spaced along the length of the cell, and their position adjusts as the cell grows (or as new carboxysomes appear) so that the spacing remains regular.  They “wiggle” due to diffusion far less than you would expect them to.  The spacing seems to be determined by the cytoskeleton, using mechanisms that were originally described in connection with the regular spacing of certain plasmids.

Why would you want to space carboxysomes so precisely?  One possibility is that it may be important to make sure that cells get equal numbers of carboxysomes when dividing.  Cyanobacteria do indeed seem to be highly egalitarian in the division of carboxysomes between daughter cells.  But if you mess up the cytoskeletal mechanisms keeping the carboxysomes in place, the distribution becomes a lot more variable.  Dave was able to see individual division events in cells lacking relevant cytoskeletal elements where one of the daughter cells received no carboxysomes at all; these cells had a severe growth disadvantage, dividing significantly later than their sisters.

Another, slightly wilder, hypothesis about why you might want to space carboxysomes as far apart from each other as possible is that they are, after all, little centers of intense chemical activity.  The RuBisCo that is stacked in almost crystalline arrays inside these organelles is vigorously converting CO2 to other molecules.  Perhaps local concentration gradients are significant, even over these short distances, and regular spacing of carboxysomes increases their efficiency.  This is not discussed in the published paper, but I understand investigations are under way.  Watch this space…