FIG. 3.

A model of phosphorylation-regulated, bipartite precursor tRNA binding by the hLa protein. The model shown here depicts bipartite interactions with a nascent pre-tRNA according to previous data (41, 67) and the model in Fig. 2. The tandem RRM-1 and -2 of the NTD mediate high-affinity binding to UUU-OH-containing RNAs. The CTD of hLa contains a basic region and an acidic region, shown as +++ and −−, respectively; residues 328 to 344 conform to a WAM, and residues 348 to 368 represent a putative PBS (see Fig. 2 and text). Together, the WAM and PBS can recognize the 5′-pppG/A motif that comprises the 5′ ends of nascent Pol III transcripts (41, 67). Serine 366, which resides in a region that demarcates a transition from basic to acidic residues, is shown as S in unphosphorylated La and as P in the phosphoserine form.