DEFECT IN EXPORT AND SYNTHESIS OF THE PERIPLASMIC GALACTOSE RECEPTOR MGLB IN DNAK MUTANTS OF ESCHERICHIA-COLI, AND DECREASED STABILITY OF THE MGLB MESSENGER-RNA

The high-affinity galactose permease, which comprises the periplasmic galactose receptor MglB, the membrane translocator MglC and the membra ne-associated ATPase MglA, displayed a reduced activity in a dnaK temp erature-sensitive mutant of Escherichia coli. This reduced transport a ctivity correlated with a reduction in the quantity of MglB. At 42 deg rees C, an accumulation of pre-MglB in the dnaK temperature-sensitive mutant reflected a defect in MglB export. In addition, an accumulation of pre-MglB in secB, secA and secY mutants suggested that SecB and th e Sec translocase are also involved in export of the periplasmic galac tose receptor. At 30 degrees C, there was no accumulation of pre-MglB in the dnaK mutant, but there was still a decreased amount of MglB in the periplasm. The reduction in MglB expression was not the result of a decrease in its stability, nor was it the result of a general defect in translation or transcription, since the MglA protein (which is exp ressed from the same operon as MglB) was synthesized in normal amounts . Two mRNAs are implicated in the expression of the mgl genes, a polyc istronic mgIBAC mRNA, and a more stable and more abundant mglB mRNA, p roduced by 3'-5' degradation of the mglBAC mRNA (R. W. Hogg, C. Voelke r & I. von Carlowitz, 1991, Mol Gen Genet 229, 453-459). The mglB mRNA is protected against exonucleases by a REP (Repetitive Extragenic Pal indrome) sequence located at its 3' extremity, which is responsible fo r the higher expression of MglB compared to MglA and MglC. The decreas ed MglB expression in the dnaK mutant at 30 degrees C in the present w ork correlated with a reduced stability of the mglB mRNA, which may ha ve resulted from a defective stabilization by the REP sequence, or fro m a defect in translation of the mglB gene.