Several small nuclear and cytoplasmic RNAs are synthesized by RNA polymerase (Pol) III as nascent transcripts that must then migrate through RNA processing, modification, and transport machinery on their trek to becoming mature, functional molecules. The first factor to interact with newly synthesized Pol III transcripts is the La protein. Evidence that this molecular chaperone accompanies precursor tRNAs through various RNA modification and processing events and facilitates the assembly of other transcripts into specific RNPs is reviewed. Although most La is nucleoplasmic, some is nucleolar, and several Pol III transcripts, including precursors of tRNAs, U6 small nuclear RNA (snRNA), RNase P RNA, and signal recognition particle (SRP) RNA, have recently been shown to move through the nucleolus. Modifications found previously on La-associated U6 snRNA, coupled with recent data, now suggest that these modifications occur in the nucleolus, while U6 RNA is associated with La. Finally, data which indicate that human La exhibits a two-step mode of RNA binding that is sensitive to phosphorylation are reviewed, and the implications of this mode of binding are considered in the context of the dynamics of RNA maturation.
RNA Pol III synthesizes many small RNAs, including 5S ribosomal rRNA and tRNAs, U6 and RNase P snRNAs, MRP small nucleolar RNA (snoRNA), as well as SRP and the family of Y small cytoplasmic RNAs. Similar to transcripts of Pols I and II, newly synthesized Pol III transcripts are subjected to a maturation process that involves removal, modification, and addition of nucleotides. Some Pol III transcripts undergo little, while others undergo extensive, processing. Precursor tRNAs endure an elaborate intranuclear maturation process that involves recognition by many cleavage and modification enzymes before transport to the cytoplasm (Wolin and Matera 1999).