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Isolation and Characterization of a Regulatory Mutant of an Aminoacyl-Transfer Ribonucleic Acid Synthetase in Escherichia coli K-12

^a Department of Molecular Biophysics, Yale University, New Haven, Connecticut 06510
Department of Biochemistry, Yale University, New Haven, Connecticut 06510
^c Department of Radiology, Yale University, New Haven, Connecticut 06510

ABSTRACT

From Escherichia coli strain K28, which is temperature sensitive for growth because of a mutation in its seryl-transfer ribonucleic acid (tRNA) synthetase gene (serS), temperature-resistant mutants were selected which were found to have a fivefold higher level of seryl-tRNA synthetase than the parent strain. The "high-level" character was found to be genetically stable and is due to a mutation in a locus denoted serO. This locus was found to be very closely linked to serS on the genetic map, and the relative gene order was concluded to be serS-serO-serC. In a serO^– strain, the normal dependence of seryl-tRNA synthetase (SerRS) activity on changes of exogenous serine concentration was not observed. In a stable heterozygous merodiploid, the serO^– mutation is still expressed, i.e., it is cis dominant. These results strongly suggest that serO is an operator site involved in the control of the serS gene.

¹ Present address: Department of Biochemistry, University of the West Indies, Kingston, Jamaica.