BioPhotovoltaics

A Brief Introduction to Synechocystis

This is a short but wide-ranging introduction to synechocystis and cyanobacteria in general. It focuses on those points that are relevant to our project and outlines the basics of photosynthesis and genetic modification in cyanobacteria. It also sketches some of the problems that we faced with using synechocystis as our organism (rather than, say, E.coli). This may be helpful to other teams. Certainly, we encountered numerous problems and were often forced to learn by trial and error. Perhaps future teams may avoid similar mistakes by reading this.

Overview

Synechocystis PCC 6803 is a species of cyanobacteria which is used as a model organism in numerous different areas of biology. Its genome was one of the first to be completely sequenced and is freely available online. Several repositories exist, of which the first was CyanoBase. Moreover, the functions of a significant proportion of its genes are understood. As such, it is simple enough to target genes that code for particular functions and it is possible to alter specific systems with considerable precision. Together with the fact that synechocystis is fairly easy to transform, this makes it an ideal organism for use in synthetic biology. Consequently, it has attracted substantial interests in the last few years as a potential producer of biofuels. Nonetheless, its use is not particularly widespread in iGEM and there are very few parts that are specific to it. As such, any team that chooses to use it faces substantial obstacles (such as the lack of suitable plasmids, promoters and ribosome binding sites).

Photosynthesis

Cyanobacteria are the only known prokaryotes that carry out plant-like oxygenic photosynthesis. It is thought that plants’ chloroplasts are, in fact, derived from endosymbiotic cyanobacteria. However, photosynthesis in cyanobacteria is unusual in several respects. In particular, the cellular machinery involved in it is organized in an atypical manner. In cyanobacteria, the electron transport chains for photosynthesis and respiration are both embedded in the same membrane system. Electrons are shuttled back and forth between the two electron transport chains and various components are involved in both processes. These are: Plastoquinone (PQ), the cytochrome b6f complex, plastocyanin (PC) and cytochrome c533. These act much like switches on a railway, shifting electrons between the two electron transport chains.

The process of photosynthesis itself is much the same as in plants. Importantly, it suffers from similar shortcomings. In particular, Rubisco is the central carbon-fixing enzyme in cyanobacteria as in plants. Rubisco is, of course, very inefficient as it has a high affinity for O2 as well as CO2. In fact, cyanobacteria have evolved a specific mechanism to combat this problem. Rubisco is sheltered within intracellular compartments called carboxysomes that concentrate CO2 and prevent oxygenation. It is currently thought that the carboxysome act as a a diffusion barrier to CO2, preventing CO2 from escaping and excluding O2. However, the carboxysome evolved millions of years ago and so is not fully adapted to modern atmospheric conditions. This means that it operates at sub-optimum efficiency. Thus, photosynthesis is not as efficient as it might be.

There are two other major rate-limiting factors that affect photosynthesis in synechocystis. First, the availability of PQ. Second, the availability of PC. Third, the availability of Ferredoxin/ NADP. The first two factors affect Photosystem I; the latter affects Photosystem II. However, there is no quick or simple way to increase the availability of any of these. Moreover, since PQ and PC are also involved in respiration, for which they are also rate-limiting factors, any gains would be split between the two processes. Thus, the best way to increase the efficiency of photosynthesis in synechocystis is to increase the availability of CO2 to Rubisco, which is the weakest link in photosynthesis.

Genetic modification and practical problems

It is relatively simple, in theory, to genetically modify synechocystis since it is naturally able to integrate exogenous DNA into its genome by homologous recombination. However, there are several practical problems associated with this process. First, synechocystis is dodecaploid. All twelve of its chromosomes must take up any foreign DNA in order for it to be fully integrated into the genome. This is a time-consuming process; It is easier to insert replicative rather than integrative plasmids into the organism. The most effective methdo for doing this is natural transformation. Of course, any genes introduced into the organism by this method cannot be passed on to future generations. Nonetheless, for projects that are time-constrained, this is a more sensible approach. It means that one can establish whether or not a particular alteration is effective without having to wait a long time. Since our project only required a proof of concept, we opted to use a replicative plasmid to insert DNA into our synechocystis samples. The lack of suitable vectors meant that our plasmid was not, in fact, BioBrick compatible – though there are some suitable plasmids listed in the registry, none of them were actually available to use.

The second major difficulty with synechocystis concerns its rate of growth. Synechocystis grow particularly slowly and so is not ideally suited to experiments with a particular time limit. This is a serious problem and we decided to try and address it by over-expressing the CmpA gene, which is involved in bicarbonate transport (see here for more detail). It has been shown that the introduction of additional bicarbonate transporters leads to a two-fold increase in carbon uptake, growth and biomass production. We hoped to achieve a similar effect by specific, carefully-targeted modification of the bicarbonate transporters. A detailed explanation of why more efficient bicarbonate transport leads to a faster rate of growth can be found on the CmpA page.

Synechocystis must also be stored under very specific conditions. In particular, it must be kept at a constant temperature of around -80C. Though many labs will have facilities capable of achieving such temperatures, some will not. We had to make use of Imperial College, London’s superior technology to store our cultures. Those teams that do not possess such facilities themselves (mostly high school teams should be aware of this.