An increased content of protease La, the lon gene product, increases protein degradation and blocks growth in Escherichia coli.

The lon gene product in Escherichia coli is an ATP-dependent protease (La) that plays an important role in the breakdown of abnormal proteins and certain normal polypeptides. Since transcription of the lon gene rises as part of the heat-shock response, we studied the physiological effects of increased levels of protease La. In cells carrying additional copies of the lon gene under the control of the lac or tac promoter, induction of the protease resulted in a rapid cessation of cell growth and in a loss of viability at stationary phase. Similarly, cells carrying a multicopy plasmid encoding the lon gene contained 2-5-fold more protease La and grew much more slowly than did control cells. In such cells, insertion sequences appeared spontaneously in the lon gene on the plasmid and prevented the excess protease production and allowed more rapid growth. The cells with increased content of protease La (due to the lon plasmid or induction of the lon gene) exhibited severalfold higher rates of degradation of abnormal proteins containing amino acid analogs and of incomplete polypeptides containing puromycin. Also, a beta-galactosidase fusion protein with enzymatic activity was relatively stable in control cells but unstable in the cells with high protease La content. In these cells, the overall degradation of normal proteins increased 2-fold, and certain cellular polypeptides appeared particularly sensitive to proteolysis. Thus, rates of protein degradation in vivo are limited in part by the cellular content of the ATP-dependent protease, and increases in transcription of the lon gene enhance proteolysis and can be deleterious to the cell.