- Carboxysomes are part of the carbon concentrating mechanism in cyanobacteria and chemoautotrophs. - Carboxysomes are a subclass of bacterial microcompartments (BMCs); BMCs can encapsulate a range of metabolic processes. - Like some viral particles, the carboxysome can be modeled as an icosahedron-in its case, having 4,000-5,000 hexameric shell subunits and 12 surface pentamers to generate curvature. - The threefold axis of symmetry of the CsoS1D protein in carboxysomes forms a pore that can open and close, allowing for selective diffusion. - Genetic modules encoding BMC shell proteins and the enzymes that they encapsulate are horizontally transferable, suggesting they enable bacteria to adapt to diverse environments.

In photosynthetic organisms, carotenoids function in light harvesting and in photoprotection. In cyanobacteria, there have been numerous reports of proteins that bind exclusively carotenoids. Perhaps the best characterized of these proteins are the 35 kDa water-soluble orange carotenoid proteins (OCPs). Structural, biochemical, and genomic data on the OCP and its paralogs are gradually revealing how these proteins function in photoprotection. paired right arrows 2004 Elsevier Inc. All rights reserved.

Bacterialmicrocompartments (BMCs) are organelles composed entirely of protein. They promote specific metabolic processes by encapsulating and colocalizing enzymes with their substrates and cofactors, by protecting vulnerable enzymes in a defined microenvironment, and by sequestering toxic or volatile intermediates. Prototypes of the BMCsare the carboxysomes of autotrophic bacteria. However, structures of similar polyhedral shape are being discovered in an ever-increasing number of heterotrophic bacteria, where they participate in the utilization of specialty carbon and energy sources.Comparative genomics reveals that the potential for this type of compartmentalization is widespread across bacterial phyla and suggests that genetic modules encoding BMCs are frequently laterally transferred among bacteria. The diverse functions of these BMCs suggest that they contribute to metabolic innovation in bacteria in a broad range of environments.

Cellular compartmentalization is a fundamental strategy through which complex tasks can be carried out, where each compartment carries out a specialized function. Compartmentalization strategies in the bacteria have (relatively) recently been discovered and are not present across the entire domain; they instead seem to be tied to niche specialization of the host species. For example, carboxysomes are necessary for all cyanobacteria to fix carbon dioxide in present-day atmospheric conditions. Carboxysomes are part of a larger family of bacterial organelles termed bacterial microcompartments (BMCs), which are all related by the proteinaceous shell that bounds the compartment.

Many different metabolic functions have been attributed to BMCs, such as carbon fixation, propanediol metabolism, and ethanolamine metabolism, and cursory genome gazing had hinted that there may be several more functional types of these organelles in a vast number of bacteria. In order to survey the functional and phylogenetic diversity of BMCs, a bioinformatic algorithm was developed to predict and categorize genetic loci that encode genes that can construct a complete organelle. Our analyses result in a taxonomy of BMCs that identifies several candidate loci and individual genes for further investigation.

We predicted many of these loci to encode for BMCs with novel functions, and examined one locus apparently isolated to the Planctomycetes and Verrucomicrobia phyla. By genetically manipulating Planctomyces limnophilus, we identified that this locus encodes a fully-functional organelle that is involved in degrading fucose and rhamnose, which is likely critical to the niche specialization of many Planctomycetes that associate with algae.

We also found the widespread usage of a novel phosphotransacetylase enzyme in the BMCs. By analyzing the primary, secondary, and quaternary structures of this novel protein, we identify well-conserved motifs within distinct domains of the protein and identify a peptide that is involved in modulating the quaternary structure.

One of the many uses of fundamental research is to provide the foundation for engineering synthetic constructs. Finally, we describe two bioengineering strategies geared towards the goal of improving the efficiency of the carbon fixation step of photosynthesis that are based on our knowledge of the cyanobacterial carbon concentrating mechanism, including the carboxysome. The first strategy focuses on the input for the carboxysome, where inorganic carbon must be transported into the cytosol. The second strategy utilizes a carboxysome assembly factor as a scaffolding mechanism.