SnRNAs are part of the spliceosome, which splices introns from mRNA. They are the fourth class of RNAs, essential for gene expression.

Although Steitz gives top ranking to her work on ribosomes, it was her seminal 1980 paper in Nature, "Are snRNPs involved in splicing?" that clinched her scientific reputation. Steitz, like many others in the early 1970s, had turned her attention to eukaryotic cells (a eukaryote is an organism whose genetic material is located within a membrane-bound nucleus). Of particular interest to Steitz was the mystery of why so much RNA was made in the nucleus, but so little—about 10 percent—ever made it out to the cytoplasm to be translated into proteins. As she tried to figure out the answer, experiments by Philip Sharp at the Massachusetts Institute of Technology (MIT), Richard Roberts at Cold Spring Harbor Laboratory, and many others coalesced into the realization in 1977 that the DNA in eukaryotic cells alternates between exons, which contain gene sequences, and introns, which do not code for any protein ("junk DNA").

Yet the discovery of introns did not explain the machinery or how all the noncoding introns were removed from a newly transcribed length of RNA. Steitz kept at the problem, and by analyzing blood samples from patients with an autoimmune disease, she and her student Michael Lerner discovered a novel entity—the snRNP. A snRNP (pronounced snurp) comprises a small length of RNA (about 150 nucleotides long) that is complexed with several proteins. "It turned out," says Steitz, "that the blood samples we analyzed contained antibodies against snRNPs.

"After we discovered snRNPs, we proposed they were involved in splicing [removing the introns from newly transcribed RNA, or pre-mRNAs, as they're now called] and we did the first experiments that showed they were, in fact, involved in splicing," says Steitz. Her lab also determined that it is a particular small nuclear RNA (snRNA), U1, in a snRNP that defines one of the splice sites of an intron via base pairing with complementary pre-mRNA.

Later, other labs coined the term spliceosome for a large assembly made up of several different snRNPs as well as additional proteins. The big mystery—why so little transcribed RNA becomes mRNA—was no longer so baffling.

Shortly after Steitz published her celebrated snRNP paper in Nature, ribozymes were discovered by Thomas Cech and by Sidney Altman of Yale and Norman Pace of Indiana University. As the name implies, these large molecules of RNA actually have the ability to catalyze a reaction—namely, they can splice or cut strands of RNA. And while it's yet to be proved, Steitz thinks that RNA catalysis is responsible for the cutting and rejoining actions of the spliceosome. "Like a standard enzyme, snRNPs come together and form a spliceosome, do their business, fall apart, and do the whole thing over somewhere else," says Steitz. If this turns out to be the case, the RNA portion of a snRNP would be considered a ribozyme.

By this time, Steitz had already advanced up the Yale ladder to become a full professor. Her lab subsequently discovered a second spliceosome that eliminates a rare class of "black sheep" introns that have atypical sequences at their splice sites.

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