Mechanism of protein chain termination: further characterization of a mutant defective in a new protein synthesis factor.

Mutant N4316 is conditionally lethal at 43 degrees . At 36 degrees it suppresses the termination codons UGA and UAA, but not the UAG codon or a missense mutant of T4 bacteriophage. In vitro, a factor rescues protein synthesis from a temperature-dependent arrest when N4316 extracts are used with RNA from bacteriophage f2. Analyses of the substrate in the arrested synthesis and of the product of the rescue reaction indicate that the factor works at the level of coat protein termination, and that it also affects the synthesis of noncoat protein products. The rescue factor is different from the release factors RF-1, RF-2, and RF-3. Model systems previously used to study release fail to score for at least one vital function in protein chain termination.