We have found earlier three small nucleolar RNA (snoRNA) species, named El, E2, and E3, that have unique nudeotide sequences and may participate in ribosome formation. The present report shows that there is a monophosphate at the 5' end of each of these three snoRNAs, suggesting that their 5' termini are formed by RNA processing. El, E2, and E3 human genomic sequences were isolated. Apparently, the E2 and E3 loci are genes for the main E2 and E3 RNA species, based on their full homology, while the El locus is a gene for an El RNA sequence variant in HeLa cells. These loci do not have any of the intragenic or flanking sequences known to be functional in other genes. The El gene is located within the first intron of the gene for RCC1, a protein that regulates onset of mitosis. There is subsntial sequence homology between the human E3 gene and flanking regions, and intron 8 and neighboring exons of the gene for mouse translation initiion factor 4AII. Injection of the human El, E2, and E3 genes into Xenopus oocytes generated sequence-specific transcripts of the approximate sizes of the respective snoRNAs. We discus why the available results are compatible with specific transcription and prosing occurring in frog oocytes.Some of the small nuclear RNA genes, such as Ul, U2, U3, U4 and U5, constitute an additional subfamily of genes transcribed by RNA polymerase II, while others, such as U6 and 7SK, represent another class of genes transcribed by RNA polymerase III (1-3). Seven small nucleolar RNAs (snoRNAs) have been described in metazoa (U3, U8, U13, U14, X, Y, and 7-2/MRP) and 12 snoRNAs have been characterized in yeast (U3, U14, snR3, snR4, snR5, snR8, snR9, snR10, snRll, snR30, snR189, and snR190) (4). We recently found three additional snoRNAs in human cells, which were named El, E2, and E3 (5). Their unique sequences imply that they are members of another class of snoRNAs, their 5' termini are not capped, they are present at low levels in vertebrate cells, and they appear to be "housekeeping" RNAs, expressed in all tissues examined (5). They may function in some aspect of ribosome formation, since in addition to being detected only in the nucleolar fraction, they are psoralen-photocrosslinked in vivo to unique segments of pre-rRNA (6). The the snoRNA plasmids when indicated) were injected into the nucleus of oocytes (20 nl per oocyte), which were then incubated at 19°C for 24 hr. When indicated, 2 hr after the plasmid injection [a-32P]GTP was injected into the cytoplasm of half of the oocytes, and then all oocytes were incubated at 19°C for 24 hr. When indicated, the solution containing one or two plasmids also included a-amanitin at 2 or 400 pg/ml ("low" or "high" level, respectively). The deproteinized samples were digested with DNase I [Worthington (code DPFF) or BRL] at 100 pg/ml, with or without RNase A (Sigma) at 40 pg/ml for either 15 min at 22°C or 30 min at 37°C. Nonradioactive RNA was fractionated by 5% (10% for radioactive RNA) polyacrylamide gel electrophoresis and electroblotted to Zeta-Pr...

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