The 7.5-kb polycistronic mop mRNA is differentially degraded in Acinetobacter calcoaceticus. The 4.9-kb 5' portion of the transcript contains the genes mopKLMNOP, encoding the multi-component phenol hydroxylase, and its 5' end decays three times faster than the 2.3-kb 3' portion encoding catechol 1,2-dioxygenase (catA). Larger amounts of the catA mRNA than the mopKLMNOP mRNA are present in the cells as a result of this processing. The site for endonucleolytic cleavage is located in the intercistronic region between mopP and catA, and contains a potential stem-loop structure and a putative RNase E cleavage site. Decay of the mop mRNA in Escherichia coli depends on RNase E. Thus, we propose that an RNase E-like activity is also present in A. calcoaceticus. Expression of MopN, one polypeptide of the multi-component phenol hydroxylase, interferes with growth of A. calcoaceticus. Thus, harmful expression of MopN may be reduced by rapid decay of its mRNA, indicating that mRNA processing contributes to differential gene expression in the large mop operon of A. calcoaceticus NCIB8250.
