Ribosomal protein L4 (RPL4) is a large ribosomal subunit protein that is structurally conserved in all kingdoms of life. This protein is a component of the 90S pre-ribosomal particle that initiates ribosomal assembly on the primary (35S) transcript. Here I show that in vivo repression of Rpl4p synthesis in S. cerevisiae results in severe loss of 60S ribosomal subunits and affects progression of the cell cycle. Analysis of rRNA processing suggests that these effects are associated with a block in the processing of the 27SA3 precursor RNA into 5.8S and 25S rRNA as well as a delay in processing of 35S precursor. More surprisingly, depletion of Rpl4p results in a unique bi-budded phenotype, with multiple cell cycle defects mainly affecting mitotic exit. To further characterize the role of RPL4 in cell cycle progression, I isolated temperature-sensitive L4 mutants. To date I have analyzed one of these mutants. Six hours after a temperature shift of this mutant, cells are uniformly arrested in SG2 progression with single large bud, unseparated genetic material, and emerging spindle apparatus. Interestingly, the mutation lies in the extended tentacles of RPL4, which is disposable in prokaryotes, but appears to play a key role in the functioning of this protein in eukaryotes. All the above data indicate an important extra-ribosomal function of RPL4 in orchestrating ribosome biogenesis with cell cycle progression in S.cerevisiae. Currently, we are attempting to define more precisely how Rpl4p is involved in these essential cellular phenomena
