have been sequenced, and others are now being 
analyzed. Functions for most minor snRNPs are still 
unknown, although the Ull snRNP has been impli- 
cated in the polyadenylation reaction that fashions 
the 3' ends of most eukaryotic messenger RNAs. 
A. Role of the human U7 snRNP in the 3 ' -end matu- 
ration of histone pre-mRNAs. One low-abundance 
snRNP, containing U7 RNA, functions in the 3 -end 
processing of histone pre-mRNAs, in which an 
endonucleolytic cut is made between two con- 
served signals in the pre-mRNA. The two conserved 
signals are recognized by distinct components of 
active nuclear extracts: the upstream stem loop 
associates with a nuclease-insensitive factor, while 
binding to the downstream element is mediated 
by the snRNP, presumably via base pairing with the 
5' end of the U7 RNA. Accordingly, the laboratory 
recently established that pre-mRNA mutations in 
the stem loop only reduce processing, whereas 
deletion of the downstream element abolishes ac- 
tivity. To delineate the exact sequence requirements 
for the interaction between the human U7 snRNP 
and the downstream conserved element. Dr. Steitz 
and her colleagues are pursuing in vitro reconsti- 
tution of active U7 snRNPs. Vectors capable of 
in vivo expression of marked and/or mutated U7 
snRNPs have also been developed and are being 
used in suppression studies with mutant histone 
genes. 
B. Structure and function of Herpesvirus saimiri- 
encoded U RNAs. It was previously established that 
marmoset T lymphocytes transformed by Herpesvi- 
rus saimiri contain four novel virus-encoded U 
RNAs [Herpesvirus saimiri U RNAs (HSURs)]. 
HSURs assemble with Sm proteins and acquire a 5' 
trimethylguanosine cap, categorizing them as typi- 
cal Sm snRNPs (of low abundance). Dr. Steitz's lab- 
oratory has now discovered a fifth HSUR, encoded 
by a nearby region of the viral genome; HSUR 5 is 
related (but not identical) to HSURs 1 and 2. These 
three HSURs share 5 '-end sequences that exhibit 
both complementarity to the AAUAAA polyadenyla- 
tion signal and homology to AU-rich sequences 
found in the 3' untranslated regions of short-lived 
mRNAs for certain lymphokines, cytokines, and 
proto-oncogenes. Therefore the possibility that 
HSURs contribute to cell transformation either by 
acting in mRNA polyadenylation or by inhibiting 
the selective degradation of (and thereby stabiliz- 
ing) important cellular messengers is being 
pursued. 
III. Other snRNPs. 
A. Analyses of the 7SK RNE The 7SK RNP is an 
abundant (2 x lOVcell) nuclear particle of un- 
known function in mammalian cells. The secondary 
structure of the 7SK RNA alone and within its RNP 
have been analyzed by chemical modification: base- 
specific reagents are used to modify selectively sin- 
gle-stranded nucleotides, which are then mapped 
by primer extension. Based on these results, oligo- 
nucleotides complementary to single-stranded re- 
gions have been identified that can degrade the 
7SK RNA with RNase H in cell extracts to test in- 
volvement in various in vitro RNA-processing reac- 
tions. Currently, hints that the 7SK RNP may be 
an auxiliary factor in splicing that transiently associ- 
ates with the U4 and/or the U6 snRNP are being 
pursued. 
B. New nucleolar snRNPs. Autoantibodies pre- 
viously known to precipitate the U3 RNP (a con- 
served, abundant nucleolar snRNP) have been used 
to isolate and characterize two additional small nu- 
cleolar RNAs from HeLa cells. Both RNAs possess a 
trimethylguanosine cap structure but are not pre- 
cipitable by anti-Sm antibodies. Sequence analysis 
has revealed that one of these RNAs (136 nucleo- 
tides long) is 85% homologous to the rodent U8 
RNA. It is therefore referred to as human US, while 
the other RNA (105 nucleotides) represents a novel 
species, U13. Human US and U13 are present at 
-20% and 5% the amount of U3 (10^/celI), respec- 
tively. Both contain two conserved sequences 
(boxes C and D), which are also present in human 
U3 RNA and in U3 RNAs from unrelated organisms. 
The role of box C and/or D in binding the common 
34 kDa autoantigen (otherwise known as 
fibrillarin) is under study. The U3, US, and U13 
RNPs appear to comprise a new subset of mamma- 
lian snRNPs, whose roles in ribosome biogenesis 
are being investigated. 
C. The human telomere transferase enzyme is an 
snRNP that synthesizes TTAGGG repeats. The large 
number of chromosomes (lO'^-lO^) generated by 
some ciliates has provided an advantageous system 
for the study of the structure, function, and biosyn- 
thesis of telomeres. Telomere terminal transferase 
(telomerase), first isolated from Tetrahymena, sur- 
prisingly turned out to be a ribonucleoprotein that 
synthesizes telomeres de novo. 
Despite the fear that telomerase activity might be 
negligible in mammalian cells (—50 chromosomes). 
Continued 
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