J. Norman Hansen
Ph.D. - University of California at Los Angeles, 1968
Professor
Department of Chemistry & Biochemistry
University of Maryland
College Park, MD 20742
Telephone: (301)-405-1847
E-mail: jh21@umail.umd.edu
Dr. Hansen's group is studying some novel ribosomally-synthesized peptide antibiotics that are produced by gram-positive bacteria. They are subtilin, which is produced by Bacilus subtilis; and nisin, which is produced by Streptococcus lactis. These antibiotic peptides contain many unusual amino acids that are introduced by post-translational modification of a precursor peptide that contains an unusual leader region that directs the secretion of the peptide through a novel secretion pathway.
These antibiotic peptides are being studied in several different ways. One is their use as a model system for testing the use of genetic engineering as a way to design and construct new antibiotics. Most common antibiotics are small organic molecules that are synthesized by multi-step enzyme pathways. Making structural variants of such antibiotics requires the use of laborious organic chemical methods. In contrast, gene-encoded antibiotic peptides can be modified by mutating their genes, whereupon the mutant analog is constructed by the protein biosynthetic machinery of the cell. One of the goals of this work is to design and construct subtilin and nisin analogs with chemical, physical, and antimicrobial properties that are superior to the natural forms of these antibiotics. This goal was recently realized by the synthesis of a mutant of subtilin in which a change of a single amino-acid residue conferred a higher specific activity and dramatically enhanced its chemical stability.
Another area of interest is the study of the genes and gene products involved in post-translation modification of the precursor peptide to the mature antibiotic. These modifications involve dehydration of serines and threonines, which react with cysteines, to form novel thioether crosslinkages. The structural gene of subtilin is found in an operon that contains other genes that are required for subtilin biosynthesis. One of these genes encodes a protein that is homologous to the multidrug resistant proteins found in humans and other mammals that transport molecules through the membrane. It is hypothesized that this protein is involved with export of subtilin outside the cell. We have established that the unusual leader sequence of the antibiotic prepeptide is required to direct proper export of subtilin. We are now in the process of defining the signals in the subtilin prepeptide that are recognized by the post-translational processing machinery. One of the intriguing possibilities is that these processing signals could be incorporated into a variety of peptides and proteins, whereupon the processing machinery will introduce the unusual amino acids. This would have the effect of extending the repertoire of amino acids that can be incorporated into peptides and proteins to include not just the standard 20 amino acids, but a variety of dehydro residues and thioether crosslinkages. These proteins would possess novel, and perhaps useful, properties.
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