THE DUPLICATED SACCHAROMYCES-CEREVISIAE GENE SSM1 ENCODES A EUKARYOTIC HOMOLOG OF THE EUBACTERIAL AND ARCHAEBACTERIAL L1 RIBOSOMAL-PROTEINS

A previously unknown Saccharomyces cerevisiae gene, SSM1a, was isolate d by screening for high-copy-number suppressors of thermosensitive mut ations in the RNA14 gene, which encodes a component from the polyadeny lation complex, The SSM1a gene codes for a 217-amino-acid protein, Ssm 1p, which is significantly homologous to eubacterial and archaebacteri al ribosomal proteins of the L1 family, Comparison of the Ssm1p amino acid sequence with that of eucaryotic polypeptides with unknown functi ons reveals that Ssm1p is the prototype of a new eucaryotic protein fa mily, Biochemical analysis shows that Ssm1p is a structural protein th at forms part of the largest 60S ribosomal subunit, which does not exi st in a pool of free proteins, SSM1a is duplicated, The second gene co py, SSM1b, is functional and codes for an identical and functionally i nterchangeable Ssm1p protein, In wild-type cells, SSM1b transcripts ac cumulate to twice the level of SSM1a transcripts, suggesting that SSM1 b is responsible for the majority of the Ssm1p pool, Haploid cells lac king both SSM1 genes are inviable, demonstrating that, in contrast wit h its Escherichia coli homolog, Ssm1p is an essential ribosomal protei n, Deletion of the most expressed SSM1b gene leads to a severe decreas e in the level of SSM1 transcript, associated with a reduced growth ra te. Polysome profile analysis suggests that the primary defect caused by the depletion in Ssm1p is at the level of translation initiation.