PHOSPHORYLATION OF THE HUMAN LA ANTIGEN ON SERINE-366 CAN REGULATE RECYCLING OF RNA-POLYMERASE-III TRANSCRIPTION COMPLEXES

The human La antigen is an RNA-binding protein that facilitates transc riptional termination and reinitiation by RNA polymerase III. Native L a protein fractionates into transcriptionally active and inactive form s that are unphosphorylated and phosphorylated at serine 366, respecti vely, as determined by enzymatic and mass spectrometric analyses. Seri ne 366 comprises a casein kinase II phosphorylation site that resides within a conserved region in the La proteins from several species. RNA synthesis from isolated transcription complexes is inhibited by casei n kinase Ii-mediated phosphorylation of La serine 366 and is reversibl e by dephosphorylation. This work demonstrates a novel mechanism of tr anscriptional control at the level of recycling of stable transcriptio n complexes.