IV. Molecular biology of S-layers

Abstract

A review with 197 references on the information available about the mol. biol. of crystalline surface layers of different bacterial groups. PS2 is the major cell wall protein of various Corynebacterium glutamicum strains. The cspB gene encoding PS2 was cloned in λ gt11 by immunol. screening and sequenced. Freeze-etching electron microscopy studies performed on the wild-type strain as well as on a disrupted strain clearly demonstrated that PS2 is responsible for the formation of a highly ordered, hexagonal array at the surface of the bacterium. Here, we also describe the composition of the S-layer of Bacillus anthracis. Two abundant surface proteins with mol. masses of 94000, named EA1 and Sap, were possible S-layer components. Their corresponding genes have been cloned. EA1 and Sap each possess three SLH motifs. EA1 is unambiguously synthesized in vivo, and could therefore be a virulence factor. This short review also presents information on the antigenic diversity of methanogenic archaea, and on some of the genes sequenced thus far that encode S-layer proteins and ABC transporters. The mechanisms inferred from the genes' organization and the proteins' sequences that might play a role in generating cell surface diversity are briefly discussed. At least three genes control the expression of the S-layer of Thermus thermophilus HB8. Gene slrA has repressor activity on the S-layer gene promoter (PslpA). Gene slpM encodes a membrane protein that functions as a transcriptional activator in vivo. The third gene is slpA itself, whose product is the S-layer protein. On this basis, we propose the existence of overlapping transcriptional and translational mechanisms which coordinately control the expression of the S-layer from T. thermophilus HB8. Campylobacter fetus strains of type A and B possess 7-8 sapA or homologues with a high degree of homol. at the 5' and 3' ends. The rearrangement by reciprocal recombination was studied. Bacillus stearothermophilus PV72 alters its surface properties in response to environmental changes. The S-layer of the wild-type B. stearothermophilus PV72 has a hexagonal (p6) symmetry and is composed of identical protein subunits (SbsA) with a mol. mass of 130000 each. When the oxygen supply is increased during continuous cultivation, SbsA becomes rapidly and irreversibly replaced by the second, smaller (mol. mass 97000) S-layer protein, SbsB, assembling into an oblique (p2) ordered lattice type. By increasing the growth temperature from 57°C to 68°C for at least 10 passages another variant, the S-layer deficient strain T5, could be isolated. The DNA sequences of the S-layer genes sbsA and sbsB of Bacillus stearothermophilus PV72 have been determined encoding a S-layer protein (mol. mass 130000) with p6 symmetry and a S-layer protein (mol. mass 96000) with p2 symmetry, resp. Both genes have been cloned and stably expressed in Escherichia coli. Recombinant S-layer fusion proteins are designed for biotechnol. applications in the areas of vaccine candidates, antibody detection systems, metabolic design and mol. machines. S-layers, which are present on the bacterial surface of several Lactobacillus species, are composed of a single protein with mol. masses between 40000 and 45000. L. acidophilus and evolutionarily closely related species contain two S-layer protein genes, only one of which is expressed. The L. acidophilus S-layer protein can be efficiently produced and secreted in L. casei. The possible role of S-layer proteins and antigenic variation in adherence will be discussed.