Prototrophic ‘revertants’ of methionine-requiring methionyl-tRNA synthetase mutants ( were examined following an observation that some of them excreted methionine. A number were found to be phenotypically indistinguishable from a class () of methionine regulatory mutants. The mutations causing reversion, like mutations, were located between a serine () and a methionine () structural gene on the linkage map, and it was concluded that the ‘suppressors’ were mutations. The ability of independently isolated mutations to cause suppression of mutants was tested and was apparently related to their ability to cause methionine overproduction. A second kind of methionine regulatory mutation () also caused suppression, while a (homoserine--transsuccinylase) mutation leading to feedback insensitivity failed to do so. Spontaneous revertants due to secondary mutations were rare, and none due to secondary mutations was detected.


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