A New Suppressor of a LamB Signal Sequence Mutation, PrlZ1, Maps to 69 Minutes on the Escherichia Coli Chromosome

Reversion analysis has been employed to isolate suppressors that restore export of a unique LamB signal sequence mutant. The mutation results in a substitution of Arg for Met at position 19, which prevents LamB export to the outer membrane and leads to a Dex- phenotype. Unlike other LamB signal sequence mutants utilized for reversion analysis, LamB19R becomes stably associated with the inner membrane in an export-specific manner. In this study, Dex+ revertants were selected and various suppressors were isolated. One of the extragenic suppressors, designated prlZ1, was chosen for further study. prlZ1 maps to 69 min on the Escherichia coli chromosome. The suppressor is dominant and SecB dependent. In addition to its effect on lamB19R, prlZ1 suppresses the export defect of signal sequence point mutations at positions 12, 15, and 16, as well as several point mutations in the maltose-binding protein signal sequence. prlZ1 does not suppress deletion mutations in either signal sequence. This pattern of suppression can be explained by interaction of a helical LamB signal sequence with the suppressor.