Self-Assembly of Nanofilaments in Cyanobacteria for Protein Co-localization.

Cyanobacteria offer great potential as alternative biotechnological hosts due to their photoautotrophic capacities. However, in comparison to established heterotrophic hosts, several key aspects, such as product titers, are still lagging behind. Nanobiotechnology is an emerging field with great potential to improve existing hosts, but so far, it has barely been explored in microbial photosynthetic systems. Here, we report the establishment of large proteinaceous nanofilaments in the unicellular model cyanobacterium Synechocystis sp. PCC 6803 and the fast-growing cyanobacterial strain Synechococcus elongatus UTEX 2973. Transmission electron microscopy and electron tomography demonstrated that expression of pduA*, encoding a modified bacterial microcompartment shell protein, led to the generation of bundles of longitudinally aligned nanofilaments in S. elongatus UTEX 2973 and shorter filamentous structures in Synechocystis sp. PCC 6803. Comparative proteomics showed that PduA* was at least 50 times more abundant than the second most abundant protein in the cell and that nanofilament assembly had only a minor impact on cellular metabolism. Finally, as a proof-of-concept for co-localization with the filaments, we targeted a fluorescent reporter protein, mCitrine, to PduA* by fusion with an encapsulation peptide that natively interacts with PduA. The establishment of nanofilaments in cyanobacterial cells is an important step toward cellular organization of heterologous pathways and the establishment of cyanobacteria as next-generation hosts.

The UV filtering potential of drop-casted layers of frustules of three diatom species.

Diatoms are in focus as biological materials for a range of photonic applications. Many of these applications would require embedding a multitude of diatoms in a matrix (e.g. paint, crème or lacquer); however, most studies on the photonic and spectral properties of diatoms frustules (silica walls) have been carried out on single cells. In this study, for the first time, we test the spectral properties of layers of frustules of three diatom species (Coscinodiscus granii, Thalassiosira punctifera and Thalassiosira pseudonana), with special focus on transmission and reflectance in the UV range. The transmittance efficiency in the UV A and B range was: T. pseudonana (56-59%) >C. granii (53-54%) >T. punctifera (18-21%) for the rinsed frustules. To investigate the underlying cause of these differences, we performed X-ray scattering analysis, measurement of layer thickness and microscopic determination of frustule nanostructures. We further tested dried intact cells in the same experimental setup. Based on these data we discuss the relative importance of crystal structure properties, nanostructure and quantity of material on the spectral properties of diatom layers. Characterization of the UV protection performance of layers of diatom frustules is of central relevance for their potential use as innovative bio-based UV filters.