|Address:||Kaul Human Genetics Building
720 20th Street South
Birmingham, AL 35294-0024
|More Information: Heflin Center for Human Genetics|
Lab Research Focus: Mobile DNA: Structure, Enzymology and Regulation
Biological development in procaryotes and eucaryotes involves a series of programmed changes in gene expression. During evolution, genes have transposed from place to place and they became integrated into temporally and developmentally regulated biochemical pathways. The principal focus of my laboratory is bacteriophage Mu, a 35,000 bp DNA virus and an efficient transposon. Mu has the ability either to transpose to a new chromosome site and form a stable quiescent lysogen or to enter a lytic cycle where it multiplies by repeatedly inserting new copies of the virus into the host genome. Exploring Mu?s biological behavior and the proteins that control genetic transposition is a powerful method to learn the underlying principles governing chromosome structure and function.
Gene Regulation. Where do transposons move in chromosomes and how do they choose new insertion points? We are developing a method based on the Taq polymerase chain reaction (PCR) technology that allows us to measure the frequency of Mu insertion at particular chromosome sites during lytic phage replication. Our results show that Mu?s movement is strongly influenced by the activity and location of proteins bound to specific sites in the chromosome. Mu prefers insertion points in the control region of genes that are not expressed compared to regions of DNA that are actively transcribed. Several host proteins described below have a strong impact on Mu transposition.
Supercoiling Proteins and DNA Structure. Bacteria do not have a true nucleus but have a negatively supercoiled DNA molecule that is wound and condensed by a small group of proteins into a folded structure?the nucleoid. DNA gyrase is the enzyme that supercoils the chromosome, and Mu has one essential gyrase binding site at its center (17,000 bp from both ends) that is essential for transposition. Our model for how this site works is that an anchored molecule of gyrase molds Mu into a highly organized interwound domain with the two ends of Mu (attL and attR) close together (see Figure).
Several small E. coli chromosomal-associated proteins also influence how often Mu transposes and the locations that it chooses as target sites. These include proteins called IHF and HNS. The IHF protein binds and bends Mu DNA specifically at a site in the operator.
This bent form of DNA is critical for proteins that bind DNA in both the lysogenic and lytic stages of virus development. HNS is the second most abundant host chromosomal protein. HNS stabilizes repressor-DNA complexes and this protein can be modified so that different forms are found when cells grow under different conditions and at different rates. HNS may control the activity of other transposons in addition to Mu.
The Mu phage system is revealing many fascinating new biochemical details about chromosome structure and genome organization. Using this novel virus, it is now possible to address evolutionary questions about the timing and selective advantage of chromosome rearrangements in growing bacterial populations.
Patrick Higgins (b. 1946) completed undergraduate studies at Wichita State University. He obtained a Ph.D. degree in the Department of Microbiology from the University of Chicago (1976), was appointed to the faculty of the University of Wyoming in 1979 and moved to Birmingham in 1984, where he is a full Professor of Biochemistry and Molecular Genetics. His research is supported through grants from the NIH, NSF, American Cancer Society, and the Parke Davis Corporation.