A cDNA expression vector encoding Drosophila ribosomal protein S14 was transfected into cultured Chinese hamster ovary (CHO) cells that harbor a recessive RPS14 emetine resistance mutation. Transformants synthesized the insect mRNA and polypeptide and consequently displayed an emetine-sensitive phenotype.These observations indicate that the insect protein was accurately expressed and correctly assembled into functional mammalian 40S ribosomal subunits.We have isolated and sequenced transcriptionally active ribosomal protein (r-protein) S14 genes from Chinese hamster ovary (CHO) cells (16, 20), human placenta (19, 21), and Drosophila melanogaster (4). The human S14 gene (RPS14) maps to chromosome 5q23-33 (15, 19), and the Chinese hamster gene maps to chromosome 2q (5, 6). The two mammalian RPS14 genes specify identical r-proteins (19,20). Drosophila RPS14 is located on the X chromosome in region 7C5-9 (4). The gene consists of a tandemly duplicated pair of transcriptionally active genes (4), both of which encode exactly the same r-protein. Insect S14 protein differs from its mammalian homologs at 20 of 151 amino acid residues. Nine of the differences involve radical amino acid replacements located almost exclusively (7 of 9) within the first 32 residues of the proteins. Complete protein-coding sequences of Chinese hamster, human, and Drosophila RPS14 can be found in GenBank under accession numbers M11241, M13934, and M21045, respectively.Nucleic acid and protein sequence similarities provide compelling evidence for stuctural homology among mammalian and Drosophila RPSJ4 loci (4,8). Because Chinese hamster RPSJ4 has been correlated with recessive emetine resistance mutations (EmtB) in somatic tissue culture cells (2,3,9,11,12,17,18,20,21), this locus offers a unique opportunity to examine conservation among eucaryotic rproteins and genes by using genetic as well as biochemical criteria.To test the function of an insect r-protein in mammalian cells, a Drosophila S14 cDNA was introduced into EmtB CHO cells under the control of a well-characterized transcriptional promoter. Previously we demonstrated that cloned human S14 genomic DNAs are transcribed and translated in EmtB rodent cells and thereby restore the emetine-sensitive phenotype (21). This was not surprising, as human and Chinese hamster S14 proteins are structurally identical. In contrast, for Drosophila S14 to function in CHO cells requires that the insect protein, whose primary structure differs from that of its mammalian homologs at several sites, be compatible with hamster cell nucleolar-transport and ribosomal subunit assembly systems and that chimeric ribosomal subunits perform efficiently in the rodent cell environment.We observed that EmtB CHO cells stably transformed with a Drosophila S14 cDNA expression vector elaborate * Corresponding author.