A large ribonuclear protein complex is required for the processing of the small-ribosomal-subunit rRNA - the small-subunit (SSU) processome [1, 2]. This preribosomal complex contains the U3 snoRNA and at least 40 proteins, which have the following properties:

• They are nucleolar.
• They are able to coimmunoprecipitate with the U3 snoRNA and Mpp10 (a protein specific to the SSU processome).
• They are required for 18S rRNA biogenesis.

There appears to be a linkage between polymerase I transcription and the formation of the SSU processome; as some, but not all, of the SSU processome components are required for pre-rRNA transcription initiation. These SSU processome components have been termed t-Utps. They form a pre-complex with pre-18S rRNA in the absence of snoRNA U3 and other SSU processome components. It has been proposed that the t-Utp complex proteins are both rDNA and rRNA binding proteins that are involved in the initiation of pre18S rRNA transcription. Initially binding to rDNA then associating with the 5' end of the nascent pre18S rRNA. The t-Utpcomplex forms the nucleus around which the rest of the SSU processome components, including snoRNA U3, assemble [3]. From electron microscopy the SSU processome may correspond to the terminal knobs visualized at the 5' ends of nascent 18S rRNA.

Utp21 is a component of the SSU processome, which is required for pre-18S rRNA processing. It interacts with Utp18 [4].

1. A large nucleolar U3 ribonucleoprotein required for 18S ribosomal RNA biogenesis. Nature 417, 967-70
2. The small-subunit processome is a ribosome assembly intermediate. Eukaryotic Cell 3, 1619-26
3. RNA polymerase I transcription and pre-rRNA processing are linked by specific SSU processome components. Genes Dev. 18, 2506-17

A large ribonuclear protein complex is required for the processing of the small-ribosomal-subunit
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