A CARBOXY-TERMINAL BASIC REGION CONTROLS RNA-POLYMERASE-III TRANSCRIPTION FACTOR ACTIVITY OF HUMAN LA PROTEIN

Human La protein has been shown to serve as a transcription factor for RNA polymerase III (pol III) by facilitating transcription terminatio n and recycling of transcription complexes. In addition, La binds to t he 3' oligo(U) ends common to all nascent pol III transcripts, and in the case of B1-Alu RNA, protects it from 3'-end processing (R. J. Mara ia, D. J. Kenan, and J. D. Keene, Mol. Cell. Biol. 14:2147-2158, 1994) . Others have previously dissected the La protein into an N-terminal d omain that binds RNA and a C-terminal domain that does not. Here, dele tion and substitution mutants of La were examined for general RNA bind ing, RNA 3'-end protection, and transcription factor activity. Althoug h some La mutants altered in a C-terminal basic region bind RNA in mob ility shift assays, they are defective in RNA 3'-end protection and do not support transcription, while one C-terminal substitution mutant i s defective only in transcription. Moreover, a C-terminal fragment lac king RNA binding activity appears able to support low levels of transc ription by pol m. While efficient multiround transcription is supporte d only by mutants that bind RNA and contain a C-terminal basic region, These analyses indicate that RNA binding contributes to but is not su fficient for La transcription factor activity and that the C-terminal domain plays a role in transcription that is distinguishable from simp le RNA binding, The transcription factor activity of La can be reversi bly inhibited by RNA, suggesting the potential for feedback inhibition of pol III transcription.