Citizens are often required to make decisions about socioscientific issues in a climate characterized by conflict within both the scientific community and the larger society. Central to the process of decisionmaking is a critical examination of the relevant scientific knowledge involved. Individuals capable of performing this task can be considered scientifically literate in a decisionmaking sense. In this article we explore two ways of critically examining scientific knowledge in the context of a current socioscientific dispute: NASA's Galileo Mission to Jupiter. The two approaches we outline, termed the positivist and social constructivist positions, are examined in terms of their inherent views concerning the nature of scientific knowledge, in particular their use of constitutive and contextual values when evaluating knowledge claims. Because the social constructivist position acknowledges the importance of contextual values, it provides citizens with accessible standards for evaluating scientific knowledge claims. The positivist position, on the other hand, relies on constitutive values which we show are normally inaccessible to ordinary citizens. The positivist position, however, is most closely associated with the predominant social issues approach to science‐technology‐society (STS) education. Implications little consensus about which statements are fact (i.e., will remain stable when challenged) and which opinion, (i.e., will be modified when challenged). All knowledge is potentially unreliable when one is dealing with a socioscientific dispute. The adoption of a social constructivist view of scientific knowledge and its inherent way of evaluating knowledge claims clearly has implications for future approaches to STS education. Although one approach might be to offer a course in the history, philosophy, and sociology of science, this would not be useful without reference to the way in which such knowledge can help students to understand the context of a conflict within the society of scientists and the larger society. As Rosenthal (1989) argues, a synthesis is needed in which social issues are seen as a vehicle for studying the social studies of science and the social issues are seen as a way of making sense of social aspects of science. However, this way of teaching STS may be difficult to implement. In British Columbia, for example, science teachers have resisted efforts to include the social context of science within a traditional university‐oriented physics course (Gaskell, 1992) and to teach a grade 11 social issues oriented sicence and technology course (Gaskell, 1989). This may be because the current social issues approach is most compatible with traditional science content as it is now taught: it simply shows the relevance of textbook knowledge (ready‐made science) to contemporary probles. The shift to the approach suggested above will require a more drastic reorganization of the curriculum, one that may be resisted by the current stakeholders in science education (Duschl, 1988; Gaskell, ...

Linker insertion mutagenesis was used to modify the paracrystalline surface layer (S-layer) protein (RsaA) of the gram-negative bacterium Caulobacter crescentus. Eleven unique BamHI linker insertions in the cloned rsaA gene were identified; at the protein level, these linker insertions introduced 4 to 6 amino acids at positions ranging from the extreme N terminus to the extreme C terminus of the 1,026-amino-acid RsaA protein. All linker-peptide insertions in the RsaA N terminus caused the secreted protein to be shed into the growth medium, suggesting that the RsaA N terminus is involved in cell surface anchoring. One linker-peptide insertion in the RsaA C terminus (amino acid 784) had no effect on S-layer biogenesis, while another (amino acid 907) disrupted secretion of the protein, suggesting that RsaA possesses a secretion signal lying C terminal to amino acid 784, near or including amino acid 907. Unlike extreme N-or C-terminal linker-peptide insertions, those more centrally located in the RsaA primary sequence had no apparent effect on S-layer biogenesis. By using a newly introduced linker-encoded restriction site, a 3 fragment of the rsaA gene encoding the last 242 C-terminal amino acids of the S-layer protein was expressed in C. crescentus from heterologous Escherichia coli lacZ transcription and translation initiation information. This C-terminal portion of RsaA was secreted into the growth medium, confirming the presence of a C-terminal secretion signal. The use of the RsaA C terminus for the secretion of heterologous proteins in C. crescentus was explored by fusing 109 amino acids of an envelope glycoprotein from infectious hematopoietic necrosis virus, a pathogen of salmonid fish, to the last 242 amino acids of the RsaA C terminus. The resulting hybrid protein was successfully secreted into the growth medium and accounted for 10% of total protein in a stationary-phase culture. Based on these results and features of the RsaA primary sequence, we propose that the C. crescentus S-layer protein is secreted by a type I secretion system, relying on a stable C-terminal secretion signal in a manner analogous to E. coli ␣-hemolysin, the first example of an S-layer protein secreted by such a pathway.The outer membrane of the dimorphic gram-negative bacterium Caulobacter crescentus is covered by a protein surface layer (S-layer) composed of a single 1,026-amino-acid protein termed RsaA (13). The S-layer is paracrystalline in nature, exhibiting an array of ring-like subunits (each composed of six copies of RsaA) arranged on a lattice with p6 symmetry and interlinked at the threefold rotational axis (57); proper crystallization of the S-layer is dependent on Ca 2ϩ ions (62, 63). The association between RsaA and the outer membrane is not completely understood, but the protein appears to be anchored to the outer membrane via noncovalent interactions with a specific smooth lipopolysaccharide (LPS) molecule (62). So far, the only known function of the C. crescentus S-layer is to protect cells against predation by a Bde...

Although S-layers are being increasingly identified on Bacteria and Archaea, it is enigmatic that in most cases S-layer function continues to elude us. In a few instances, S-layers have been shown to be virulence factors on pathogens (e.g. Campylobacter fetus ssp. fetus and Aeromonas salmonicida), protective against Bdellovibrio, a depository for surface-exposed enzymes (e.g. Bacillus stearothermophilus), shape-determining agents (e.g. Thermoproteus tenax) and nucleation factors for fine-grain mineral development (e.g. Synechococcus GL 24). Yet, for the vast majority of S-layered bacteria, the natural function of these crystalline arrays continues to be evasive. The following review up-dates the functional basis of S-layers and describes such diverse topics as the effect of S-layers on the Gram stain, bacteriophage adsorption in lactobacilli, phagocytosis by human polymorphonuclear leukocytes, the adhesion of a high-molecular-mass amylase, outer membrane porosity, and the secretion of extracellular enzymes of Thermoanaerobacterium. In addition, the functional aspect of calcium on the Caulobacter S-layer is explained.

SummaryThe paracrystalline surface (S)-layer of Caulobacter crescentus is composed of a single secreted protein (RsaA) that interlocks in a hexagonal pattern to completely envelop the bacterium. Using a genetic approach, we inserted a 12 amino acid peptide from Pseudomonas aeruginosa strain K pilin at numerous semirandom positions in RsaA. We then used an immunological screen to identify those sites that presented the inserted pilin peptide on the C. crescentus cell surface as a part of the S-layer. Eleven such sites (widely separated in the primary sequence) were identified, demonstrating for the first time that S-layers can be readily exploited as carrier proteins to display 'epitope-size' heterologous peptides on bacterial cell surfaces. Whereas intact RsaA molecules carrying a pilin peptide could always be found on the surface of C. crescentus regardless of the particular insertion site, introduction of the pilin peptide at 9 of the 11 sites resulted in some proteolytic cleavage of RsaA. Two types of proteolytic phenomena were observed. The first was characterized by a single cleavage within the pilin peptide insert with both fragments of the S-layer protein remaining anchored to the outer membrane. The other proteolytic phenomenon was characterized by cleavage of the S-layer protein at a point distant from the site of the pilin peptide insertion. This cleavage always occurred at the same location in RsaA regardless of the particular insertion site, yielding a surface-anchored 26 kDa proteolytic fragment bearing the RsaA N-terminus; the C-terminal cleavage product carrying the pilin peptide was released into the growth medium. When the results of this work were combined with the results of a previous study, the RsaA primary sequence could be divided into three regions with respect to the location of a peptide insertion and its effect on S-layer biogenesis: (i) insertions in the extreme N-terminus of RsaA either produce no apparent effect on S-layer biogenesis or disrupt surface-anchoring of the protein; (ii) insertions in the extreme C-terminus either produce no apparent effect on S-layer biogenesis or disrupt protein secretion; and (iii) insertions more centrally located in the protein either have no apparent effect on S-layer biogenesis or result in proteolytic cleavage of RsaA. These data are discussed in relation to our previous assignment of the RsaA N-and C-terminus as regions that are important for surface anchoring and secretion respectively.

Although S-layers are being increasingly identified on Bacteria and Archaea, it is enigmatic that in most cases S-layer function continues to elude us. In a few instances, S-layers have been shown to be virulence factors on pathogens (e.g. Campylobacter fetus ssp. fetus and Aeromonas salmonicida), protective against Bdellovibrio, a depository for surface-exposed enzymes (e.g. Bacillus stearothermophilus), shape-determining agents (e.g. Thermoproteus tenax) and nucleation factors for fine-grain mineral development (e.g. Synechococcus GL 24). Yet, for the vast majority of S-layered bacteria, the natural function of these crystalline arrays continues to be evasive. The following review up-dates the functional basis of S-layers and describes such diverse topics as the effect of S-layers on the Gram stain, bacteriophage adsorption in lactobacilli, phagocytosis by human polymorphonuclear leukocytes, the adhesion of a high-molecular-mass amylase, outer membrane porosity, and the secretion of extracellular enzymes of Thermoanaerobacterium. In addition, the functional aspect of calcium on the Caulobacter S-layer is explained.