Reinventing RNA Polymerase
Transcription, the process of generating RNA from DNA lies at the heart of cellular growth and maintenance. Transcription is a complex process usually involving several molecular components and yet is highly regulated. Amazingly, the T7 bacteriophage accomplishes this task using only one protein. This protein, the T7 RNA polymerase (RNAP) first recognizes and binds to a specific sequence of DNA called the promoter and initiates polymerization of the RNA while holding on to the promoter sequence. However, in this initial phase T7 RNAP is highly inefficient and undertakes abortive synthesis of small RNA (2-10 nucleotides long). Eventually, this RNAP escapes this abortive phase releasing the promoter DNA contacts and transforms into a fast and highly processive polymerase (Elongation phase). All RNA polymerases undergo through similar phases that involve binding and unbinding of transcription co-factors. But the question remained how does a single protein undergoes such drastic transition and completely reinvents itself. Comparison of the X-ray crystallographic structures of the T7 RNA polymerase complex in these two phases showed large differences in the N-terminal domain of the protein (Yin and Steitz Science (2002)).
Professor Taekjip Ha and his graduate student, Rahul Roy in collaboration with Dr. Smita Patel and her post-doc Guo-Qing Tang at Department of Biochemistry at Robert Wood Johnson Medical School, have now been able to observe this reinvention of the RNA polymerase in real time for single DNA-RNAP-RNA complexes. They show that this transition can occur over a wide range of RNA lengths but it is mostly complete as RNA length reaches 12 nucleotides.
Read more about the results in Tang et al. PNAS (2009).