This project's goal is the isolation of frameshift mutants and their suppressors in mouse L-cells. Frameshift mutations, which can be induced by planar heterocyclic ring structures containing polyamine side chains (ICR compounds), are usually the result of single base additions ("+1") or deletion ("+1") in the DNA strand. This leads to a messenger RNA with a shifted reading frame that changes the coding specificity and often generates a premature chain-termination codon: The "+1" frameshifts can be corrected or suppressed secondary mutations in transfer RNA genes. These suppressor tRNAs can read a 4-base codon instead of the normal 3-base codon, thereby restoring the normal reading frame. Thus like temperature sensitivity or nonsense suppression, frameshift suppression is a system of conditional lethality in which the phenotype of the mutation is either wild-type or mutant depending upon the conditions, in this case the presence or absence of a suppressor. Frameshift mutants have low reversion rates in the absence of mutagens and the frameshift fragment should be separable from the wild type protein due to its smaller size. Frameshift mutations have been isolated in bacteria and in yeast, a eucaryote. In vitro assays are being developed for "+1" frameshift suppressors. These assays involve translation of rabbit globin or ovalbumin message in the presence of known frameshift suppressors from yeast or bacteria. Specific base sequences occur in these messages which should be subject to "suppression" by frameshift suppressors, resulting in shifts in the normal reading frame and the generation of a chain-termination codon. The protein product which is shorter then the wild-type protein is detected by gel electrophoresis. Transfer RNA extracted from putative frameshift suppressor lines of mouse L-cells will be screened for suppression activity using these assays. The target enzyme for the isolation of mutants will be hypoxanthine-guanine phosphoribosyl transferase (HGPRT). Selective procedures for isolating mutants and revertants of this enzyme are well established.