Synthetic Biology in Cyanobacteria and Applications for Biotechnology

This chapter discusses the state of the art of synthetic biology in cyanobacteria as well as application areas for synthetic biology within cyanobacteria biotechnology. Our ability to insert or change genes in cyanobacteria, and to control expression of heterologous genes, has advanced significantly over the past decade. Control of transcription by external signal molecules is robust and reliable for model strains, in particular, Synechocystis PCC 6803 where even multiple, orthogonal signals can be used. Of particular importance is an ability to tightly repress gene expression in the absence of an inducer, as heterologous enzymes often impose a growth burden on cells when expressed. Control of gene expression via internal or environmental signals, such as light or other nutrients, is less refined. Control of mRNA translation, for example, with riboswitches or trans-acting RNAs, is particularly useful when combined with transcriptional repression but is sparsely used. Gene editing, using CRISPR/Cas or other nucleases, has been demonstrated in several strains but requires standardization to become widespread. Integration of genes into the genome is common, but expression from the genome suffers can be weak, location dependent, and growth-phase dependent. Expression of genes from episomal plasmids is strong, and there are recent efforts at standardization of these, but plasmids require selection pressure such as antibiotics to maintain for long (>3 weeks) cultivations. Two goals of synthetic biology in cyanobacteria in the near future are to diversify signal inputs via characterization of new transduction proteins and to link multiple signal inputs into a circuit, so that fast, reliable, condition-dependent gene expression can be achieved.