Relationship Between Messenger Ribonucleic Acid and Enzyme Levels Specified by the Leucine Operon of Escherichia coli K-12

Abstract

The levels of leucine-forming enzymes in E. coli K-12 varied over a several thousand-fold range, depending upon conditions of growth. The highest levels were achieved by growing auxotrophs in a chemostat under condtions of leucine limitation. Under such conditions, enzyme levels were increased 45- to 90-fold relative to cells grown in minimal medium containing leucine (the latter values arbitrarily called 1). Leucine operon-specific mRNA levels were elevated to about the same extent as enzyme levels in cells grown in a chemostat. Growth in media of greater complexity resulted in progressively lower levels of leucine-forming enzymes, reaching a value of less than 0.02 for growth in a medium containing tryptone broth and yeast extract. The levels of leucine operon-specified enzymes and mRNA were also measured in strains containing about 25 copies of plasmid pCV1(ColE1-leu)/chromosome. For such strains grown in minimal medium, enzyme levels were proportional to the number of plasmids per cell, and they followed the same trends as those described above upon derepression in a chemostat or upon repression following growth in rich media. Leucine mRNA, measured by pulse-labeling and hybridization-competition experiments, was roughly proportional to enzyme levels over this entire range. For a plasmid-containing strain grown in a chemostat under conditions of leucine limitation (about 100 plasmids/chromosome), about 27% of pulse-labeled RNA was coded for by genes in or adjacent to the leucine operon, and 10% of the total protein was .beta.-isopropylmalate dehydrogenase.