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 temperature-sensitive mutant of This reduced transport activity correlated with a reduction in the quantity of MgIB. At 42 C, an accumulation of pre-MgIB in the temperature-sensitive mutant reflected a defect in MgIB export. In addition, an accumulation of pre-MgIB in and mutants suggested that SecB and the Sec translocase are also involved in export of the periplasmic galactose receptor. At 30 C, there was no accumulation of pre-MgIB in the 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 genes, a polycistronic mRNA, and a more stable and more abundant mRNA, produced by 3′-5′ degradation of the mRNA (R. W. Hogg, C. Voelker & I. von Carlowitz, 1991, 229, 453-459). The 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 mutant at 30 C in the present work correlated with a reduced stability of the mRNA, which may have resulted from a defective stabilization by the REP sequence, or from a defect in translation of the gene.


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