YEAST RIBOSOMAL-PROTEIN L1 IS REQUIRED FOR THE STABILITY OF NEWLY SYNTHESIZED 5S RIBOSOMAL-RNA AND THE ASSEMBLY OF 60S RIBOSOMAL-SUBUNITS

Ribosomal protein L1 from Saccharomyces cerevisiae binds 5S rRNA and c an be released from intact 60S ribosomal subunits as an L1-5S ribonucl eoprotein (RNP) particle. To understand the nature of the interaction between L1 and 5S rRNA and to assess the role of L1 in ribosome assemb ly and function, we cloned the RPL1 gene encoding L1. We have shown th at RPL1 is an essential single-copy gene. A conditional null mutant in which the only copy of RPL1 is under control of the repressible GAL1 promoter was constructed. Depletion of L1 causes instability of newly synthesized 5S rRNA in vivo. Cells depleted of L1 no longer assemble 6 0S ribosomal subunits, indicating that L1 is required for assembly of stable 60S ribosomal subunits but not 40S ribosomal subunits. An L1-5S RNP particle not associated with ribosomal particles was detected by coimmunoprecipitation of L1 and 5S rRNA. This pool of L1-5S RNP remain ed stable even upon cessation of 60S ribosomal subunit assembly by dep letion of another ribosomal protein, L16. Preliminary results suggest that transcription of RPL1 is not autogenously regulated by L1.