Where to begin? Carbon-fixing organelle assembles from the inside-out.

Research Areas:

Cell biology, Cyanobacteria, Organelle formation, Live cell imaging, Synechococcus elongatus

Imaging Needs:

Spatial resolution over time, ability to conduct FRAP

First Imaging System:
  • Nikon Ti inverted microscope
  • 100mW solid state 488nm laser
  • Photonics Instruments MicroPoint laser targeting system
  • 100x 1.4 numerical aperture objective lens
  • Lumen Dynamics EXFO XL-120 fluorescence light source
  • Hamamatsu ORCA-R2 CCD camera
Second Imaging System:
  • Nikon TE-2000 microscope
  • 100x 1.4 numerical aperture objective lens
  • Lumencor LED fluorescence illuminator
  • Hamamatsu ORCA-ER CCD camera
  • ImageJ software
Imaging Bacteria

For the resolution and sensitivities required to visualize bacterial organelles and proteins, researchers look to Hamamastu's ORCA cameras, and also the ImageEM series. Find out how Ashley Cadby and colleagues visualize cell wall growth in bacteria using Hamamatsu’s ImagEM camera. Read now.

THE QUESTION

How do cyanobacteria form carboxysomes?

Plants are not unique in their ability to sequester CO2, converting this greenhouse gas into biologically usable forms like sugars. Cyanobacteria also play a significant role in the global carbon cycle. They process CO2 inside a microcompartment called the carboxysome, which contains a densely packed cargo of the carbon sequestering enzyme ribulose-1,5-bisphosphate carboxylase (RuBisCO) and carbonic anhydrase, surrounded by an icosahedral protein shell.

THE BARRIERS

The role of carboxysomes in carbon fixation is well understood, but the mechanism and time sequence of their formation was unknown. Electron cryotomography has provided glimpses of partially assembled carboxysomes that always include both RuBisCO and shell proteins—indicating the enzyme might have some capacity to self-assemble. Indeed, RuBisCO has been shown to self-associate in vitro without shell proteins. The shell proteins, in turn, can form capsules absent the cargo enzyme. So which comes first, the contents or the shell?

THE SOLUTION

The Bacterial Carbon-Fixing Organelle Is Formed by Shell Envelopment of Preassembled Cargo
Anna H. Chen, Avi Robinson-Mosher, David F. Savage, Pamela A. Silver, and Jessica K. Polka
PloS One. 2013; 8(9): e76127. PMCID: PMC3762834.

To answer this question, Chen, et al,1 used FRAP—fluorescence recovery after photobleaching—to watch Synechococcus elongatus bacteria form carboxysomes. The images and video at 7 to 12 frames per second show that new carboxysomes generally assemble one at a time, beginning from a small “daughter” seed of compact RuBisCO enzyme. The RuBisCO seed itself emerges from a mature “mother” carboxysome. It grows spontaneously in the cytoplasm, adding RuBisCo molecule by molecule, until reaching a critical size. Then, abruptly, the protein coat forms. The authors propose that the enclosing protein coat stops further growth of the organelle and accounts for the consistent size observed in carboxysomes.

THE POSSIBILITIES

Chen, et al,1 used Hamamatsu’s ORCA-ER CCD camera to watch carboxysome biogenesis over time in living cyanobacteria. Find out what other microbiologists are discovering with Hamamatsu’s cameras—read Exciting Insights into Cell Growth.

References

  1. Chen, et al. The Bacterial Carbon-Fixing Organelle Is Formed by Shell Envelopment of Preassembled Cargo. PloS One. 2013 Jan 8(9): e76127. PMCID: PMC3762834.
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