mologous stretches of nucleotides reside in regions 
that can be successfully targeted by RNase H for de- 
struction of the U8 snRNP particle. Injection of com- 
plementary deoxyribonucleotides into the Xenopus 
oocyte is therefore being used to ask whether the U8 
snRNP (like U3) contributes to pre-rRNA process- 
ing. Hints that U8 is involved in a step affecting 
the appearance of 28S rRNA have recently been 
obtained. 
A gene for U15 resides within an intron of the 
rtbosomal protein S3 gene. Two variants of U15 
RNA (formerly called X) , which are not trimethyl- 
guanosine-capped but bind fibrillarin, have been 
detected in human cells. Curiously, the gene for one 
of these variants — instead of being present in multi- 
ple copies, as for other U RNA genes — is single- 
copy. It resides within an intron of the ribosomal 
protein S3 gene and is encoded on the same strand as 
the mRNA. Cell-free extracts can process U15 from 
intron transcripts to yield the same 5' terminus as 
native U15, arguing that the nucleolar small RNA 
and S3 mRNA are both derived from the S3 pre- 
mRNA. The presence of the gene for a small nucleo- 
lar RNA within an intron of a ribosomal protein gene 
links the production of these two contributors to 
ribosome biogenesis in mammalian cells. 
The work described in the previous three sections 
is supported by a grant from the National Institute of 
General Medical Sciences, National Institutes of 
Health. 
Viral snRNPs 
Some viruses encode small RNAs that assemble to- 
gether with host proteins to form snRNPs related to 
cellular particles. Such RNAs from both Herpesvirus 
saimiri and Epstein-Barr virus are currently under 
study (supported by a grant from the Cancer Insti- 
tute, National Institutes of Health). They appear to 
contribute to cellular transformation, since they are 
among the relatively few viral gene products ex- 
pressed in virus-transformed lymphocytes. 
Herpesvirus szimiri-encoded U RNAs may stabi- 
lize short-lived cellular messages. Marmoset T lym- 
phocytes transformed by H. saimiri contain seven 
novel virus-encoded U RNAs called HSURs (//. sai- 
miri U RNAs). HSURs assemble with Sm proteins 
and acquire a 5'-trimethylguanosine cap, categoriz- 
ing them as Sm snRNPs, but are of low abundance 
(-750-7500 the level of Ul RNA). Three of 
the HSURs share 5'-end sequences that exhibit 
homology to AU-rich sequences found in the 
3'-untranslated regions of short-lived mRNAs 
for certain lymphokines, cytokines, and proto- 
oncogenes. The possibility that HSURs contribute to 
cell transformation by inhibiting the selective degra- 
dation of (and thereby stabilizing) important cellu- 
lar messengers is supported by in vitro ultraviolet- 
crosslinking studies, which have revealed that the 
AU-rich sequence at the 5' ends of HSURs 1,2, and 5 
binds the same 32-kDa protein as the degradation 
signals of several oncogene and growth factor 
mRNAs. Attempts to purify and clone this protein 
are under way, with the goal of elucidating its func- 
tion in mRNA degradation. 
Epstein-Barr virus-encoded RNA specifically 
binds ribosomal protein L22. Human B lympho- 
cytes latently infected with Epstein-Barr virus pro- 
duce large amounts (107cell) of two snRNAs called 
EBERs (Epstein-Barr-encoded RNAs). The EBERs are 
transcribed by RNA polymerase III (pol III) and are 
permanently associated with the La protein, an au- 
toantigen that binds, at least transiently, to all pol 111 
transcripts through an association with their U-rich 
tails. EBER 1 was recently demonstrated to bind a 
second highly abundant cellular protein (15 kDa) 
named EAP (for EBER-associated protein). The clon- 
ing and sequencing of a human EAP cDNA revealed 
that EAP is 77% identical to a previously character- 
ized sea urchin protein that is developmentally ex- 
pressed but of unknown function. Recently antibod- 
ies against EAP were raised and used to determine 
EAP's binding site on EBER 1 and its subcellular lo- 
calization. EAP binds to a particular (conserved) 
stem-loop of EBER 1, recognizing nucleotides in 
both the single- and double-stranded regions. Immu- 
nofluorescent staining of nucleoli and the cyto- 
plasm by anti-EAP antibodies led to the realization 
that EAP may be a ribosomal protein. Indeed, it has 
been identified as the large subunit polypeptide 
L22, in collaboration with Dr. Ira Wool (University 
of Chicago) . Studies are now under way to elucidate 
why binding a significant fraction (~50%) of a spe- 
cific ribosomal protein in the infected cell is impor- 
tant to Epstein-Barr virus. 
Dr. Steitz is also Henry Ford H Professor of Mo- 
lecular Biophysics and Biochemistry at Yale Uni- 
versity School of Medicine. 
Articles 
Baserga, S.J., Gilmore-Hebert, M., and Yang, X.W. 
1992. Distinct molecular signals for nuclear im- 
port of the nucleolar snRNA, U3. Genes Dev 
6:1120-1130. 
Baserga, S J. , Yang, X.W. , and Steitz, J.A. 1 99 1 . An 
intact Box C sequence in the U3 snRNA is re- 
quired for binding of fibrillarin, the protein com- 
mon to the major family of nucleolar snRNPs. 
EMBO f 10:2645-2651. 
GENETICS 265 
