Dr. Englund's research for many years has concerned the biochemistry and molecular genetics of parasites known as African trypanosomes. This parasite, Trypanosoma brucei, causes sleeping sickness in humans and related disease in livestock. These parasites are studied not only because they are pathogens but also because they have a fascinating biology. For example, their mitochondrial genome is kinetoplast DNA (kDNA), composed of several thousand 1 kb minicircles and a few dozen 23 kb maxicircles, all catenated together in a giant planar network. The network, within the mitochondrial matrix, is condensed into a highly organized disk-shaped structure. The disk is attached to the flagellar basal body by a trans-membrane filament system,
Over the years his lab has focused mainly on the kDNA replication mechanism and we now understand this process in considerable detail (see ref. 1 for recent review). Key features are the topoisomerase-catalyzed release of minicircles from the network to form free minicircles. These undergo replication and the progeny are reattached to the network edge. They have studied many proteins involved in this process that are organized in specific sites surrounding or within the kDNA, like a replication machine. They recently discovered, in collaboration with C.C. Wang, a mitochondrial proteasome-like protease. Knockdown of the protease by RNAi causes minicircles and maxicircles to replicate out of control until the kinetoplast is sometimes larger than the nucleus. They postulated that the protease function is to degrade positive regulators of kDNA synthesis. They have discovered the regulator for maxicircles which is a PIF1-related helicase. When the helicase is degraded by the protease or knocked down by RNAi, maxicircles disappear. When the protease is inactivated (e.g. by RNAi) or if the helicase is overexpressed, then maxicircles increase dramatically. Thus, the level of helicase controls maxicircle replication. This system will require much more study before it can be fully appreciated, and one function may be to regulate the size of the network and the ratio of minicircles to maxicircles.
In previous studies on GPI anchoring, we discovered the GPI precursor and its biosynthetic pathway, the first in any organism. These studies, in collaboration with Jerry Hart, were facilitated by the fact that the trypanosome makes more GPIs than any other cell type.
We also identified the remodeling pathway that incorporates myristate into trypanosome GPIs. In searching for myristate's source, we discovered (contrary to a 30-year dogma) that trypanosomes do indeed make fatty acids (predominantly myristate) de novo. We then found that the fatty acid synthesis machinery differed fundamentally from type 1 or 2 pathways used by all other organisms. Instead, trypanosomes use an unprecedented mechanism involving microsomal fatty acid elongases, a system whose regulation leads to synthesis of different products required in different vector or host environments. Myristate is needed only for the life cycle stage in the mammalian bloodstream.
Other contributions from our lab include the discovery, with Don Crothers, of sequence-directed DNA bending in a kDNA fragment. We also constructed pZJM, a widely used trypanosome RNAi vector and we made the first RNAi library in any organism.
1. Jensen, R.E. and Englund, P.T. (2012) Network News: The replication of kinetoplast DNA. Ann. Rev. Microbiol. 66,473-491.