genetic mechanism for the oscillation of a- versus 
p-pili expression was sought; the a- and P-pili sub- 
unit genes were found to arise from a single genetic 
locus by a process of chromosomal rearrangement 
that occurs as an inversion event in which the 
end parts of the inverted segment are within the 
pili subunit structural genes. Thus M. bovis pos- 
sesses a genetic mechanism for pili expression that 
restricts the antigenic repertoire of each strain to 
two types only. When these findings are considered 
in evolutionary terms, it seems likely that a primor- 
dial pili gene of the 7V-methylphenylalanine class 
entered several different bacterial species; once es- 
tablished in each species, distinctive mechanisms 
for the production of antigenic diversity arose as 
a consequence of the ecology of the organism 
and the selective pressures of the host immune 
response. 
B. Characterization of an invasin protein of 
Yersinia enterocolitica. Invasion of epithelial cells is 
an important step in the pathogenic strategy of 
many bacterial species. In two of the three species 
comprising the genus Yersinia (Y. pseudotuberculo- 
sis and Y. enterocolitica) the invasive phenotype is 
confined by a single outer membrane protein, in- 
vasin. In studies carried out collaboratively with the 
laboratory of Dr. Stanley Falkow (Stanford Univer- 
sity), the gene coding for the invasin of Y en- 
terocolitica was cloned, sequenced, and the topog- 
raphy of the protein on the surface of the 
bacterium determined. The gene for invasin codes 
for an 835-amino acid protein; the use of antibod- 
ies to synthetic peptides corresponding to various 
regions of this sequence to search for surface-ex- 
posed domains of invasin indicated that the amino- 
terminal and carboxyl-terminal domains of the pro- 
tein are embedded within the outer membrane, 
leaving a centrally located, surface-exposed loop 
containing the receptor binding domain; within 
this loop there exists a serine protease-sensitive re- 
gion that is normally cleaved during the invasive 
process. 
Studies to define the cellular receptor for invasin, 
to determine the minimal sequence constituting 
the receptor binding domain, and to learn how in- 
PUBLICATIONS 
vasin initiates phagocytosis by all eukaryotic cells 
(except erythrocytes) are being conducted. 
II. Immunochemistry of Small Peptides. 
Small, synthetic peptides containing amino acid 
sequences found in microbial proteins have been 
proposed as vaccines for the prevention of a re- 
markable variety of viral, bacterial, and parasitic in- 
fectious diseases. However, in practice most such 
vaccines lack efficacy; although they elicit antisera 
that bind the immunizing peptide with high affinity, 
the affinity of the same sera for the target protein is 
often substantially lower, apparently below the af- 
finity required to confer a protective effect. An im- 
munochemical explanation for this phenomenon 
was sought, using as a model system antisera elic- 
ited to a peptide corresponding to a 13-amino acid 
segment of the central a-helix of bovine 
calmodulin. The results of these studies indicate 
that antibodies were elicited to at least two distinct 
regions of the peptide. Antibodies of one specificity 
within the polyclonal immune response to the pep- 
tide do not require the carboxyl-terminal two 
amino acids of the peptide for antigenic recogni- 
tion, are present in the sera in relatively low con- 
centrations, and are able to bind the target protein 
(calmodulin) with high affinity. Antibodies of the 
other antibody specificity require atomic informa- 
tion contributed by the carboxyl-terminal two resi- 
dues of the peptide, are present in the sera in rela- 
tively high concentrations, but bind the target 
protein with low affinity. The implications of this 
study for the development of peptide vaccines stem 
from the recognition that within a polyclonal, anti- 
peptide immune response may exist a small popu- 
lation of antibodies that cross-react with the target 
protein with high affinity. Studies are being con- 
ducted on the construction of peptide im- 
munogens designed to maximize the proportion of 
useful antibodies within a polyclonal immune re- 
sponse. 
Dr. Schoolnik is also Associate Professor of Medi- 
cine and of Microbiology and Immunology at the 
Stanford University School of Medicine. 
Books and Chapters of Books 
Relman, D., Schoolnik, G.K. , Swartz, M.N., and Weller, EE 1988. Syphilis and nonvenereal treponematoses. 
In Scientific American Medicine (Rubenstein, E., and Federman, D.D., Eds.). New York: Carol Peckham 
Snow, vol VI, pp 1-10. 
Continued 
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