5' PROCESSING OF TRANSFER-RNA PRECURSORS CAN BE MODULATED BY THE HUMAN LA ANTIGEN PHOSPHOPROTEIN

Eukaryotic precursor (pre)-tRNAs are processed at both ends prior to m aturation. Pre-tRNAs and other nascent transcripts synthesized by RNA polymerase III are bound at their 3' ends at the sequence motif UUUOH [3' oligo(U)] by the La antigen, a conserved phosphoprotein whose role in RNA processing has been associated previously with 3'-end maturati on only. We show that in addition to its role in tRNA 3'-end maturatio n, human La protein can also modulate 5' processing of pre-tRNAs. Both the La antigen's N-terminal RNA-binding domain and its C-terminal bas ic region are required for attenuation of pre-tRNA 5' processing. RNA binding and nuclease protection assays with a variety of pre-tRNA subs trates and mutant La proteins indicate that 5' protection is a highly selective activity of La. This activity is dependent on 3' oligo(U) in the pre-tRNA for interaction with the N-terminal RNA binding domain o f La and interaction of the C-terminal basic region of La with the 5' triphosphate end of nascent pre-tRNA. Phosphorylation of La is known t o occur on serine 366, adjacent to the C-terminal basic region. We sho w that this modification interferes with the La antigen's ability to p rotect pre-tRNA(i)(Met) from 5' processing either by HeLa extract or p urified RNase P but that it does not affect interaction with the 3' en d of pre-tRNA. These findings provide the first evidence to indicate t hat tRNA 5'-end maturation may be regulated in eukaryotes. Implication s of triphosphate recognition is discussed as is a role for La phospho protein in controlling transcriptional and posttranscriptional events in the biogenesis of polymerase III transcripts.