An alternative protein factor which binds the internal promoter ofXenopus5S ribosomal RNA genes

Abstract
In small oocytes of Xenopus species, two sets of 5S RNA genes, oocytetype and somatic-type, are fully activated. The 5S RNA transcripts are temporarily stored, half in association with TFIIIA to form a 7S particle, the other half in association with tRNA and two proteins (p48 and p43) to form a 42S particle. It has been established previously that TFIIIA binds to the intenal control region of JS RNA genes and promotes their transcription. Here we show that protein can be translocated from the 42S particles to 5S RNA genes, but only after treatment of the particles with ribonuclease. Nevertheless, once transferred, stable protein-DNA complexes are formed and DNase-protection experiments ahow that binding is specific to the gene promoter, covering exactly the same sequence as TFIIIA. The DNA-binding protein is identified as p48 which, after isolation by ion-exchange chromatography, wil1 bind to 5S RNA genes in the absence of ribonuclease.