The precise sequence of events leading to the initiation of rRNA synthesis is not certain. Until recently, the model was that UBF binds to the ribosomal DNA (rDNA) first and then helps to recruit SL1/TIFIB. However, Joost C. B. M. Zomerdijk and coworkers have now demonstrated that UBF itself does not bind stably to rDNA but instead rapidly associates and dissociates. They further demonstrate that SL1/TIFIB to stabilize the binding of UBF to the rDNA promoter. Based on these findings, they question the idea that UBF activates transcription through recruitment of SL1/TIFIB at the rDNA promoter and instead propose alternative model in which SL1/TIFIB directs the formation of the pre-initiation complex by binding to the core promoter, helps to stabilize the binding of UBF and then acts together with UBF to recruit RNA polymerase I .
    Once the pre-initiation complex has assembled the RNA polymerase I machinery is ready to initiate transcription. DNA flanking the transcription initiation site melts to create a small bubble that allows nucleotides to align with the template strand and the first phosphodiester bond is formed. The transcription bubble grows longer as RNA polymerase I moves along the DNA so that the transcription bubble eventually becomes 12 to 23 bp long. RNA polymerase I moves down the template strand, leaving UBF and SL1 behind. At least one additional factor, elongation factor SIL, associates with the transcription elongation complex. Thus, elongation factor SⅡ participates in transcription elongation by both RNA polymerases I and Ⅱ. The elongation rate for human RNA polymerase I has been estimated to be about 95 nucleotides/s. Transcription elongation continues until the RNA polymerase I machinery reaches the transcription terminator. Then transcription is terminated and the transcript and RNA polymerase I are released. The released polymerase is free to reinitiate transcription from a previously activated and engaged promoter with pre-bound UBF and SL1. The continued presence of UBF and SL1 at the promoter allows RNA polymerase I to initiate a new round of transcription much more rapidly than would otherwise be possible. Initiation has been estimated to occur at a minimum of once every five seconds in vivo.
    RNA polymerase I transcription terminates at the 3＇end of the gene at specific sequences. Transcription termination requires the assistance of a terminator protein (transcription termination factor 1 or TTF-1 in mammals) and an RNA polymerase I transcription release factor (PTRF). The termination signal or terminator contains two elements. The arrangement is similar, but not identical, in other eukaryotes. The upstream T-rich element codes for the last 10 to 12 nucleotides in the terminated transcript. The downstream terminator protein binding element (AGGTCGACCAGA/TT/ANTCCG) is called the Sal box because it contains GTCGAC the Sal I restriction endonuclease cleavage site. The Sal box must be present in the orientation shown for termination to take place. When the RNA polymerase I transcription elongation complex encounters the transcription termination factorⅠ bound to the Sal box, the complex pauses and the RNA polymerase I transcription release factor interacts with both RNA polymerase I and TTF-1. Following release, an exonuclease trims the 3＇-end of the transcript to produce the mature 3＇-end. Finally, RNA polymerase I is released. The RNA polymerase I transcription termination mechanism in other eukaryotes is similar to that described for mammals. However, transcript dissociation does not appear to require a release factor in yeast. Instead, release seems to depend entirely on the instability of the nascent RNA-DNA hybrid in the active polymerase.