PRLA SUPPRESSION OF DEFECTIVE EXPORT OF MALTOSE-BINDING PROTEIN IN SECB MUTANTS OF ESCHERICHIA-COLI

An Escherichia coli strain containing a signal sequence mutation in th e periplasmic maltose-binding protein (MBP) (malE18-1) and a point mut ation in the soluble export factor SecB (secBL75Q) is completely defec tive in export of MBP and unable to grow on maltose (Mal- phenotype). We isolated 95 spontaneous Mal+ revertants and characterized them gene tically. Three types of extragenic suppressors were identified: inform ational (missense) suppressors, a bypass suppressor conferring the Mal + phenotype in the absence of MBP, and suppressors affecting the prlA gene, which encodes a component of the protein export apparatus. In th is study, a novel prlA allele, designated prlA1001 and mapping in the putative second transmembrane domain of the PrlA (SecY) protein, was f ound. In addition, we isolated a mutation designated prlA1024 which is identical to prlA4-2, the mutation responsible for the signal sequenc e suppression in the prlA4 (prlA4-1 prlA4-2) double mutant (T. Sako an d T. lino, J. Bacteriol. 170:5389-5391, 1988). Comparison of the prlA1 024 mutant and the prlA4 double mutant provides a possible explanation for the isolation of these prlA alleles.