Additional low-abundance human small nuclear ribonucleoproteins: U11, U12, etc.

Two-dimensional gel fractionation has revealed the existence of a number (greater than or equal to 8) of additional species of HeLa cell small RNAs that have 5' trimethylguanosine cap structures and are bound by proteins containing Sm epitopes. Therefore, these low-abundance (10(3)-10(4) per cell) RNAs belong to the Sm class of small nuclear ribonucleoproteins (snRNPs), whose best-known members are the four highly abundant (approximately 10(6) per cell) particles required for pre-mRNA splicing. The complexity of Sm snRNPs in mammalian cells is thus not greatly different from that previously established for lower eukaryotes. Two of the new RNAs, designated U11 (131 nucleotides) and U12 (150 nucleotides), have been sequenced. The U11 and U12 snRNPs have been characterized further by examining their nuclease sensitivity and their possible interactions with other snRNPs. Potential roles for the low-abundance snRNPs in aspects of pre-mRNA processing are discussed.