A yeast homolog of chromatin assembly factor 1 is involved in early ribosome assembly

Cells have a recurrent need for the correct assembly of protein-nucleic aci d complexes. We have studied a yeast homolog of the smallest subunit of chr omatin assembly factor 1 (CAM), encoded by YMR131c and termed "RRB1" [1]. U nlike other yeast homologs, Msi1p, and Hat2p, Rrb1p is essential for cell v iability. Impairment of Rrb1p function results in decreased levels of free 60S ribosomal subunits and the appearance of half-mer polysomes, suggesting its involvement in ribosome biogenesis. Using tandem affinity purification (TAP [2]) combined with mass spectrometry, we show that Rrb1p is associate d with ribosomal protein L3. A fraction of Rrb1p is also found in a protein -precursor rRNA complex containing at least ten other early-assembling ribo somal proteins. We propose that Rrb1p is required for proper assembly of pr eribosomal particles during early ribosome biogenesis, presumably by target ing L3 onto the 35S precursor rRNA. This action may resemble the mechanism by which CAM assembles histones H3/H4 onto newly replicated DNA.