Lysinibacillus sphaericus JG-B53 was isolated from the uranium mining waste pile Haberland near Johanngeorgenstadt, Germany. Previous studies have shown that many bacteria that have been isolated from these heavy metal contaminated environments possess surface layer (S-layer) proteins that enable the bacteria to survive by binding metals with high affinity. Conversely, essential trace elements are able to cross the filter layer and reach the interior of the cell. This is especially true of the S-layer of L. sphaericus JG-B53, which possesses outstanding recrystallization and metal-binding properties. In this study, S-layer protein gene sequences encoded in the genome of L. sphaericus JG-B53 were identified using next-generation sequencing technology followed by bioinformatic analyses. The genome of L. sphaericus JG-B53 encodes at least eight putative S-layer protein genes with distinct differences. Using mRNA analysis the expression of the putative S-layer protein genes was studied. The functional S-layer protein B53 Slp1 was identified as the dominantly expressed S-layer protein in L. sphaericus JG-B53 by mRNA studies, SDS-PAGE and N-terminal sequencing. B53 Slp1 is characterized by square lattice symmetry and a molecular mass of 116 kDa. The S-layer protein B53 Slp1 shows a high similarity to the functional S-layer protein of L. sphaericus JG-A12, which was isolated from the same uranium mining waste pile Haberland and has been described by previous research. These similarities indicate horizontal gene transfer and DNA rearrangements between these bacteria. The presence of multiple S-layer gene copies may enable the bacterial strains to quickly adapt to changing environments.

S-layer proteins provide a biocompatible environment with different kinds of functional groups, perfect for the sequential coupling of any kind of biofunctional molecule. In addition, their nanostructure and their ability to crystallize on surfaces in a nanometer-thick monolayer ensure a regular arrangement of these molecules on solid supports. In this work, a thrombin-binding aptamer and an ofloxacin-binding aptamer were coupled with different chemical crosslinkers to S-layer proteins using them for defined immobilization. S-layer protein monomers and paracrystalline S-layers were successfully modified with the thrombin-binding aptamer. However, S-layer protein monomers were not able to crystallize after aptamer modification and showed no thrombin binding during random surface attachment. In contrast, aptamers linked to an intact S-layer in suspension or an S-layer coating were still functional. The modification rate of S-layers with the thrombin-binding aptamer was estimated with one aptamer to two unit cells (unit cell = four monomers). Verification of the functionality of both aptamers through target binding after S-layer-immobilization on solid supports was proven by laser-induced fluorescence spectroscopy (LIFS), resonant mirror sensor (IAsys), and quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. Hence, this study presents S-layer proteins as an interesting alternative to existing immobilization matrices for recognition biomolecules.

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