Build and tailor microbial sunscreen molecules.
Photosynthetic pigments are primarily designed to harvest light in the 400-700 nm range and thus do not protect organisms from the harmful effects of UV-radiation, or light in the solar spectrum that is less than 400 nm in wavelength. This type of light causes damage to nucleic acids, proteins and other metabolites in cells and is known to effect processes such as tetrapyrrole synthesis, nitrogen fixation, carbon fixation, energy transfer in photosynthesis, and cell differentiation1. Therefore, some cyanobacteria produce secondary metabolites, referred to as microbial sunscreens, to absorb UV-light1. One example is scytonemin, a compound that strongly 315-400 nm light due to its unusual homodimeric structure derived from the coupling of tyrosine and tryptophan. Scytonemin has received much interest because in addition to its biological role as a UV-absorbing compound2, it has anti-inflammatory and anti-proliferative properties3,4. Based on scytonemin derivatives isolated from Nature, it appears that the scaffold of scytonemin, similar to chlorophyll (project 1), is a rich target for modification enzymes5,6.
The biosynthetic gene cluster for assembling scytonemin was first described for Nostoc punctiforme (Figure 2a)7. Although much has been elucidated about the biosynthesis of this interesting compound7-10, key aspects of scytonemin biosynthesis remain mysterious, including the proteins involved in dimerization and derivative formation (Figure 2b). Therefore, we will investigate non-conserved genes found inserted into the scytonemin biosynthetic pathways of different organisms. We hypothesize these genes encode proteins involved in modifying the scytonemin scaffold to produce derivatives, or libraries of compounds that share a common scaffold, but exhibit altered chemical functionality.
Figure 2. Scytonemin biosynthesis. (a) The N. punctiforme gene cluster contains conserved genes that encode copies of tryptophan (trpECABD) and tyrosine (tyrA) biosynthetic enzymes7, others that have been biochemically shown to encode proteins that assemble the monomer units (scyA-C)9-10, and those originally thought to be involved in dimerization (scyD-F)8. Of the remaining seven conserved genes, five are of unknown function (Np5232-5236) and two are involved in pathway regulation8. (b) ScyA-C build monomer units9-10, but the proteins involved in dimerization and derivative formation are unknown.
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5. Bultel-Ponce, V., Felix-Theodose, F., Sarthou, C., Ponge, J. F. & Bodo, B. New pigments from the terrestrial cyanobacterium Scytonema sp. collected on the Mitaraka inselberg, French Guyana. J Nat Prod 67, 678-681, doi:10.1021/np034031u (2004).
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7. Soule, T. et al. A comparative genomics approach to understanding the biosynthesis of the sunscreen scytonemin in cyanobacteria. BMC Genomics 10, 336, doi:10.1186/1471-2164-10-336 (2009).
8. Soule, T., Garcia-Pichel, F. & Stout, V. Gene expression patterns associated with the biosynthesis of the sunscreen scytonemin in Nostoc punctiforme ATCC 29133 in response to UVA radiation. J Bacteriol 191, 4639-4646, doi:10.1128/JB.00134-09 (2009).
9. Balskus, E. P. & Walsh, C. T. Investigating the initial steps in the biosynthesis of cyanobacterial sunscreen scytonemin. J Am Chem Soc 130, 15260-15261, doi:10.1021/ja807192u (2008).
10. Balskus, E. P. & Walsh, C. T. An enzymatic cyclopentyl[b]indole formation involved in scytonemin biosynthesis. J Am Chem Soc 131, 14648-14649, doi:10.1021/ja906752u (2009).