Genetic studies of sup-9, unc-93, and sup-10 strongly suggest that these genes encode components of a multi-subunit protein complex that coordinates muscle contraction in Caenorhabditis elegans. We cloned sup-9 and sup-10 and found that they encode a two-pore K ϩ channel and a novel transmembrane protein, respectively. We also found that UNC-93 and SUP-10 colocalize with SUP-9 within muscle cells, and that UNC-93 is a member of a novel multigene family that is conserved among C. elegans, Drosophila, and humans. Our results indicate that SUP-9 and perhaps other two-pore K ϩ channels function as multiprotein complexes, and that UNC-93 and SUP-10 likely define new classes of ion channel regulatory proteins.

To elucidate the molecular mechanism of frameshift suppression by the SUF2 gene ofyeast, the sequences ofDNA fragments carrying the SUF2-1 and suf2 + alleles of the gene and surrounding regions have been determined. Comparison of the suppressor and wild-type sequences indicates that the SUF2 gene product is a proline tRNA. Disregarding possible base modifications, we find that the wild-type suf2 + anticodon of the tRNA inferred from the DNA sequence is 3'-GGA-5'. The SUF2-1 mutation represents the insertion of a G-C base pair at a position in the gene that corresponds to the anticodon loop of the tRNA. Replacement ofthe wild-type suf2 + anticodon by a 3'-GGGA-5' fourbase anticodon enables the SUF2-1 tRNA to suppress the 5'-CCCU-3' four-base codons generated as the result of the his4-712 and hi&4-713 frameshift mutations. This nontriplet codon-anticodon interaction restores the correct reading frame and allows synthesis of a functional his4 protein.Frameshift ihutations result from addition or deletion of base pairs in a gene encoding a protein product. Mutations of this type usually render the gene product nonfunctional due to inability of the translational apparatus to recognize the shift in reading frame. This results in an incorrect specification ofamino acids and in termination of translation at the first out-of-phase nonsense codon encountered.The correct reading frame can be restored in strains carrying frameshift mutations by various compensatory mechanisms. In some instances, suppressor mutations in genes external to that which carries the frameshift mutation have been shown to affect the structures of tRNAs, tRNA base-modification enzymes, or ribosomal proteins (1-3). Altered tRNAs containing an extra base in the anticodon have been implicated in suppression of + 1 frameshift mutations (1). Although such tRNAs may be capable of reading four bases rather than the normal three bases, the exact mechanism governing four-base translocation on the ribosome is uncertain (4). In the case of altered tRNA modification enzymes and ribosomal proteins, the molecular mechanisms of frameshift suppression are unknown.A complete molecular analysis of nontriplet decoding interactions, typified by suppression offrameshift mutations, can be expected to provide a more detailed view of the normal in vivo decoding mechanism and of the mechanisms for translational control of protein synthesis. To examine this problem in the lower eukaryote Saccharomyces cerevisiae, mutationally induced nontriplet reading systems have been developed in our laboratory by the isolation of external suppressors of frameshift mutations at the his4 locus. Suppressor mutations mapping at 25 different loci have been identified (5-8).Six of these suppressors have been shown to suppress the +1 G-C insertion mutations his4-712 and his4-713 (7, 9). The altered mRNAs produced in strains carrying these suppressible mutations contain a 5'-CCCU-3' four-base sequence in place ofa wild-type 5'-CCU-3' proline codon (9). Despite the fact that these his4 m...

No abstract