Mrt4 is a nucleolar component of the ribosome assembly machinery that shares notable similarity and competes for binding to the 25S rRNA GAR domain with the ribosomal protein P0. Here, we show that loss of function of either P0 or Mrt4 results in a deficit in 60S subunits, which is apparently due to impaired rRNA processing of 27S precursors. Mrt4, which shuttles between the nucleus and the cytoplasm, defines medium pre-60S particles. In contrast, P0 is absent from medium but present in late/cytoplasmic pre-60S complexes. The absence of Mrt4 notably increased the amount of P0 in nuclear Nop7-TAP complexes and causes P0 assembly to medium pre-60S particles. Upon P0 depletion, Mrt4 is relocated to the cytoplasm within aberrant 60S subunits. We conclude that Mrt4 controls the position and timing of P0 assembly. In turn, P0 is required for the release of Mrt4 and exchanges with this factor at the cytoplasm. Our results also suggest other P0 assembly alternatives.
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