Alp, a suppressor of lon protease mutants in Escherichia coli.

Escherichia coli lon mutants lack a major ATP-dependent protease, are sensitive to UV light and methylmethane sulfonate (MMS), and overproduce capsular polysaccharide. Evidence is presented that an activity (Alp), cloned on a multicopy plasmid, can suppress the phenotypes of lon mutants. The sensitivity to UV and MMS is a reflection of the stabilization of the cell division inhibitor SulA, while the capsule overproduction arises through the stabilization of a transcriptional activator of capsule biosynthetic genes, RcsA. Multicopy alp (pAlp) suppressed capsule formation in delta lon cells, and delta lon cells containing the pAlp plasmid were resistant to MMS treatment. The MMS resistance of delta lon pAlp+ cells correlates with an increase in the degradation of SulA to that found in lon+ cells. Lon-directed degradation of SulA was energy dependent, as was the increase in degradation of SulA in delta lon pAlp+ cells. alp maps close to pheA, at 57 min on the E. coli chromosome. Although pAlp can substitute for Lon, cells lacking alp activity did not have the phenotype on a lon mutant. This study demonstrates that at least one activity, when overproduced in the cell, can substitute for Lon protease.