Background-Changes in myocardial microstructure are important components of the tissue response to infarction but are difficult to resolve with current imaging techniques. A novel technique, diffusion spectrum MRI tractography (DSI-tractography), was thus used to image myofiber architecture in normal and infarcted myocardium. Unlike diffusion tensor imaging, DSI-tractography resolves multiple myofiber populations per voxel, thus generating accurate 3D tractograms, which we present in the myocardium for the first time.
Persistent infection by papillomaviruses involves the maintenance of viral DNA as a nuclear plasmid, the replication of which requires host DNA polymerases. The role of the cellular DNA polymerase a-primase holoenzyme was probed by using soluble extracts from rodent cells that replicate bovine papilloma virus 1 and human papilloma virus 6b DNA in the presence of the viral El helicase and the E2 transcription factor. Monoclonal antibodies directed against the catalytic 180-kDa subunit of polymerase a inhibit DNA synthesis in this system. Addition of purified human polymerase a-primase holoenzyme to neutralized extracts restores their DNA synthetic activity. The amino-terminal 424 amino acids ofEl forms a specific protein complex with the p180 polymerase subunit. Immune complexes can be isolated with antibodies directed against El that contain a DNA polymerase activity. Moreover, this polymerase activity can be neutralized by anti-polymerase a antibodies. Permissivity barriers were not encountered in this in vitro system, as bovine El can interface with the murine and human replication apparatus. Although the large tumor antigens encoded by simian virus 40 and polyoma share limited primary sequence homology with the papillomavirus El proteins, the organization of functional motifs at the level of primary protein structure is remarkably similar. In addition to their origin-specific DNA-binding activity, each of these helicases may function to help recruit the cellular polymerase primase complex to the viral replication origin.A central event in the initiation of DNA synthesis is the localization of one or more DNA polymerases (pols) to the origin of replication (ori). This localization appears to be accomplished through a series ofprotein-protein interactions in a variety of biological systems (for review, see ref. 1). Although these interactions have been well documented at the genetic and biochemical level in bacteria and their phages, little is known about how the DNA pols of higher eukaryotes recognize cellular oris. Although oris of higher eukaryotes, along with the presumed initiator proteins that recognize these elements, are ill defined, it is possible to examine how cellular DNA pols are targeted to the wellcharacterized oris of DNA tumor viruses (2-4).The papovaviruses constitute a family of small, transforming DNA viruses that exhibit a complete dependence on host-cell functions to replicate (5). Each virus encodes an initiator helicase capable of assembling a specific nucleoprotein structure on the viral ori and unwinding it (for review, see refs. 5 and 6). In the case of simian virus 40 (SV40) and polyoma, the viral large tumor antigen (T) associates with the cellular pol a-primase, and this interaction may aid in localizing pol a to the ori (2-4, 7, 8). DNA pol a is responsible for initiating bidirectional DNA synthesis from the SV40 ori in vitro (9).Papillomaviruses (PVs) behave as regulated replicons. Rodent cells transformed by bovine papilloma virus (BPV) 1 maintain the viral DNA as a mult...
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