Reid Johnson, a professor in the Biological Chemistry Department, studies chromosome structure and function in microorganisms. Dr. Johnson has had a long term interest in specialized DNA recombination reactions, especially site-specific DNA recombination reactions. In 1985, he developed an in vitro system for dissecting a DNA inversion reaction that regulates flagellin biosynthesis in Salmonella. This recombination reaction enables the microbial pathogen to escape a host immune response by changing the antigenic character of its major surface protein. Mechanistic investigations into the Salmonella DNA inversion reaction led to the discovery of recombinational enhancer elements and a global regulatory protein Fis that mediates enhancer activity. The Johnson lab recently demonstrated that DNA exchange is mediated by a translocation of recombinase subunits that are covalently-linked to the cleaved DNA ends within the synaptic complex and that the Fis/enhancer system controls the direction and processivity of this subunit rotation reaction. The subunit rotation mechanism is likely to operate for all members of the serine recombinase family but is unlike DNA exchange reactions in other recombination systems.
The Fis protein is one of a small group of abundant DNA bending proteins in bacteria that have diverse roles in regulating recombination, transcription, and replication reactions as well as in chromosome packaging. In addition to investigating the control of specific DNA transactions by nucleoid proteins, the Johnson lab is currently emphasizing their role in chromosome organization. The laboratory recently solved a series of X-ray structures of Fis bound to different curved DNA sequences and, together with biophysicist John Marko, discovered that Fis condenses chromosomes in vitro by stabilizing DNA loops. These activities imply that Fis may be a prominent factor responsible for establishing the looped-domain structure of the bacterial chromosome. Research on bacterial chromosome binding proteins has been extended to the abundant HMGB class of chromatin-associated proteins in eukaryotic cells. Earlier work elucidated the mode of DNA binding and bending, in part through an NMR-based structure of a DNA complex of an HMGB protein from the yeast S. cerevisiae in collaboration with Juli Feigon?s laboratory at UCLA. Current efforts are focused on the in vivo binding specificity and function of HMGB proteins in yeast.
Reid C. Johnson is a Professor of Biological Chemistry in the UCLA School of Medicine, where he joined the faculty in 1986. He received his Ph.D. from the University of Wisconsin-Madison in 1983 and did postdoctoral work at Caltech. Dr. Johnson has had a long standing research interest in mechanisms and control of specialized DNA recombination reactions, such as transposition and site-specific recombination. An additional focus of his laboratory has concerned the varied functions of abundant nucleoid-associated DNA bending proteins in regulating transcription reactions and chromosome behavior in microorganisms. This work has recently been extended to include HMGB chromatin-associated proteins in eukaryotic cells, especially yeast.
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