RESUMO
The eukaryotic RNA polymerases Pol I, Pol II, and Pol III are the central multiprotein machines that synthesize ribosomal, messenger, and transfer RNA, respectively. Here we provide a catalog of available structural information for these three enzymes. Most structural data have been accumulated for Pol II and its functional complexes. These studies have provided insights into many aspects of the transcription mechanism, including initiation at promoter DNA, elongation of the mRNA chain, tunability of the polymerase active site, which supports RNA synthesis and cleavage, and the response of Pol II to DNA lesions. Detailed structural studies of Pol I and Pol III were reported recently and showed that the active center region and core enzymes are similar to Pol II and that strong structural differences on the surfaces account for gene class-specific functions.
Assuntos
RNA Polimerases Dirigidas por DNA , Modelos Químicos , Modelos Genéticos , RNA/biossíntese , RNA/química , Transcrição Gênica/genética , Animais , RNA Polimerases Dirigidas por DNA/química , RNA Polimerases Dirigidas por DNA/genética , RNA Polimerases Dirigidas por DNA/ultraestrutura , Ativação Enzimática , Humanos , Modelos Moleculares , RNA/genéticaRESUMO
A computer-based system has been developed for the integration and display of computerized tomography (CT) image data in the operating microscope in the correct perspective without requiring a stereotaxic frame. Spatial registration of the CT image data is accomplished by determination of the position of the operating microscope as its focal point is brought to each of three CT-imaged fiducial markers on the scalp. Monitoring of subsequent microscope positions allows appropriate reformatting of CT data into a common coordinate system. The position of the freely moveable microscope is determined by a non-imaging ultrasonic range-finder consisting of three spark gaps attached to the microscope and three microphones on a rigid support in the operating room. Measurement of the acoustic impulse transit times from the spark gaps to the microphones enables calculation of those distances and unique determination of the microscope position. The CT data are reformatted into a plane and orientation corresponding to the microscope's focal plane or to a deeper parallel plane if required. This reformatted information is then projected into the optics of the operating microscope using a miniature cathode ray tube and a beam splitter. The operating surgeon sees the CT information (such as a tumor boundary) superimposed upon the operating field in proper position, orientation, and scale.
