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Defect in export and synthesis of the periplasmic galactose receptor MgIB in dnaK mutants of Escherichia coli, and decreased stability of the mgIB mRN

Génétique et Biochimie, Institut Jacques Monod, Université Paris 7, 2 place Jussieu, 75251 Paris Cedex 05, France

²Author for correspondence: Gilbert Richarme. Tel: +33 1 44275098. Fax: +33 1 44273580.

ABSTRACT

The high-affinity galactose permease, which comprises the periplasmic galactose receptor MgIB, the membrane translocator MgIC and the membrane-associated ATPase MgIA, displayed a reduced activity in a dnaK temperature-sensitive mutant of Escherichia coli. This reduced transport activity correlated with a reduction in the quantity of MgIB. At 42 ⁰C, an accumulation of pre-MgIB in the dnaK temperature-sensitive mutant reflected a defect in MgIB export. In addition, an accumulation of pre-MgIB in secB, secA and secY mutants suggested that SecB and the Sec translocase are also involved in export of the periplasmic galactose receptor. At 30 ⁰C, there was no accumulation of pre-MgIB in the dnaK mutant, but there was still a decreased amount of MgIB in the periplasm. The reduction in MgIB 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 MgIA protein (which is expressed from the same operon as MgIB) was synthesized in normal amounts. Two mRNAs are implicated in the expression of the mgl genes, a polycistronic mgIBAC mRNA, and a more stable and more abundant mgIB mRNA, produced by 3'-5' degradation of the mgIBAC mRNA (R. W. Hogg, C. Voelker & I. von Carlowitz, 1991, Mol Gen Genet 229, 453-459). The mgIB mRNA is protected against exonucleases by a REP (Repetitive Extragenic Palindrome) sequence located at its 3' extremity, which is responsible for the higher expression of MgIB compared to MgIA and MgIC. The decreased MgIB expression in the dnaK mutant at 30 ⁰C in the present work correlated with a reduced stability of the mgIB mRNA, which may have resulted from a defective stabilization by the REP sequence, or from a defect in translation of the mgIB gene.

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