Nils Kröger Assistant Professor Office: MSE 2100P Phone: 404-894-4228 Fax: 404-894-7452 E-mail Nils Kröger Kröger Research Group

B.Sc., Philipps University of Marburg (Germany); M.Sc., University of Regensburg (Germany); Ph.D., University of Regensburg

Research Interests

Biomineralization. The formation of inorganic materials under the control of a living organism (biomineralization) is a widespread biological phenomenon, which evolved on our planet about 600 million years ago. Microalgae like Diatoms, Synurophytes (both SiO₂ formers) and Coccolithophores (CaCO₃ formers) are among the most remarkable biomineral forming organisms representing unicells capable of producing intricately ornamented, nanostructured minerals. Microalgal biomineralization is therefore regarded as paradigm for the controlled fabrication of nanopatterned inorganic materials. Since biomineral structure is a species-specific characteristic, the blueprint information for biomineral morphogenesis must be encoded within the organism's genomes. Biomineralization research aims to identify and isolate the protein-guided, cellular machinery that executes this remarkable genetic program. This task is greatly aided by the recent genome sequencing projects for a Diatom (Thalassiosira pseudonana) and a Coccolithophore (Emiliania huxleyi). Understanding the molecular mechanism that enables unicellular organisms to perform biomineralization will allow the development of novel, biomimetic syntheses for the production of nanostructured inorganic materials.

Silica Biotechnology. Silica formation by Diatoms is a very rapid, highly controlled process that takes place within a specialized intracellular compartment termed the silica deposition vesicle (SDV). Recently, novel phosphoproteins (silaffins) and unusually long-chain polyamines have been identified and implicated in Diatom biosilica formation. Experiments in vitro have shown that combinations of silaffins and long-chain polyamines spontaneously form supramolecular assemblies ( organic matrix ) that dramatically speed up silica formation from monosilicic acid solutions. Remarkably, the structure of the silica produced critically depends on the type of silaffin present within the organic matrix. Therefore, it is expected that changing the silaffin equipment of a Diatom cell by gene technology should result in novel biosilica nanopatterns. Previously, genetic transformation of Diatoms has been established, thus opening the possibility to introduce into a Diatom's genome mutated or foreign silaffin genes, as well as shutting off the expression of specific endogenous silaffins. These interferences are expected to affect the properties of the organic matrix inside the SDV of the transformed cells resulting in altered biosilica nanopatterns. Research in Silica Biotechnology aims to establish the molecular tools allowing the creation of mutated Diatoms that produce tailored silica nanostructures adapted for nanotechnological applications.

Posttranslational Modification of Silaffins. Biosilica forming proteins from Diatoms (silaffins) exhibit extremely complex chemical structures. After ribosomal translation of the silaffin genes most of the polypeptides' amino acids become modified by the attachment of alkyl-chains to lysine residues, hydroxylation of specific proline residues as well as phosphorylation, glycosylation and sulfation of almost all hydroxyl groups. Future research aims to solve the chemical structures of silaffins and to elucidate the function of individual protein domains regarding biosilica formation. The remarkable machinery that catalyzes the posttranslational modification of silaffins is expected to consist of unconventional enzymes that are not yet known from any other organisms. Identifying these modifying enzymes will provide important clues with respect to regulation and evolution of biosilica formation.

Representative Publications

Poulsen, N. and N. Kröger (2005). A new molecular tool for transgenic diatoms: Control of mRNA and protein biosynthsis by an inducible promoter-terminator cassette. FEBS J. 272, 3413-3423.

E. V. Ambrust et al. (2004). The genome of the diatom Thalassiosira pseudonana: ecology, evolution, and metabolism. Science 306, 79-86.

Poulsen, N., and N. Kröger (2004). Silica morphogenesis by alternative processing of silaffins in the Diatom Thalassiosira pseudonana. J. Biol. Chem., 279, 42993-42999.

Sumper, M. and N. Kröger (2004). Silica formation in Diatoms: the function of long-chain polyamines and silaffins. J. Mater. Chem. 14, 2059-2065.

Poulsen, N., M. Sumper and N. Kröger (2003). Biosilica formation in Diatoms: Characterization of native silaffin-2 and its role in silica morphogenesis. Proc. Natl. Acad. Sci. USA 100, 12075-12080.

Kröger, N., Lorenz, S., Brunner, E. and Sumper, M. (2002) Biosilica morphogenesis requires silaffin phosophorylation. Science 298, 584-586.

Kröger, N., Deutzmann, R. and Sumper, M. (2001). Silica precipitating peptides from Diatoms: The chemical structure of silaffin-1A from Cylindrotheca fusiformis. J. Biol. Chem. 276, 26066-26070.

Kröger, N., Deutzmann, R., Bergsdorf, C. and Sumper, M. (2000). Species specific polyamines from Diatoms control silica morphology. Proc. Natl. Acad. Sci. USA 97, 14133-14138.

Kröger, N., Deutzmann, R., and Sumper, M. (1999). Polycationic peptides from Diatom biosilica that direct silica nanosphere formation. Science 286, 1129-1132.