Cyanobacterial Genetics: Rapid evolution by Lateral Gene Transfer?

Dr Dean Price (RSBS)
in association with
Dr Susan Howitt (BaMBi),

The Price lab (RSBS) is studying photosynthetic acquisition of inorganic carbon (CO2 and HCO3-) in several cyanobacterial species. This efficient adaptation is known as a CO2 concentrating mechanism (CCM) since it results in elevation of CO2 around the primary carboxylase, Rubisco, allowing photosynthetic CO2 fixation to occur even when the external concentration of CO2 would normally limit growth. The cyanobacterial CCM features active transporters for CO2 and HCO3-, and a micro-organelle known as the carboxysome which acts as the site of CO2 elevation; many of these systems have been genetically and physiologically identified and discovered in the Price lab. Although cyanobacteria have existed for at least 2.5 billions years it appears likely that the present day CCM in cyanobacteria arose as little as 300-400 million years ago during the largest decline in past atmospheric CO2 levels. Cyanobacteria have a natural ability to take-up and integrate large chunks of foreign DNA into their genomes, and so it is possible that parts of the CCM may have been borrowed form some non-photosynthetic bacteria, further evolved, and passed on to other cyanobacteria.

Genetic transfer of genes from the divergent a and ß classes of cyanobacteria
Cyanobacteria can be divided into two major groups based on evolutionary phylogeny and differences in the CCM gene types. The slow growing, deep sea species have been termed a-cyanobacteria because they have a set of carboxysomes genes that are completely different from the ß-cyanobacteria. The latter species are predominantly found in freshwater environments. Interestingly, the carboxysome genes in a-cyanobacteria are very similar to those found in non-photosynthetic sulphur bacteria suggesting that they may have been acquire by lateral gene transfer. This project asks how likely this lateral gene transfer is by undertaking an experiment to transfer genes for a-carboxysomes in an engineered ß-cyanobacterium that lacks all the genes for ß-carboxysomes. Are the foreign genes transcribed and do they assemble into functional carboxysome? This approach could also be applied to genes coding for different types of CO2 and HCO3- transporters.

This project would involve the construction of plasmid expression vectors and introduction into strains lacking carboxysomes genes or genes for major CO2 and HCO3- transporters. Functional analysis of mutants will include measurement of HCO3- uptake using membrane inlet mass spectrometry and verification of gene transcription. The project will utilize both molecular biology (mutagenesis, DNA sequencing, PCR) and biochemistry (transport assays, membrane isolation and Western blotting) and Protein mass spectrometry for identification of expressed proteins.

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