RNA processing is involved in the post-transcriptional control of the citQRP operon from Lactococcus lactis biovar diacetylactis

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Abstract
The importance of Lactococcus lactis biovar diacetylactis (L. diacetylactis) in the dairy industry is due to its ability to produce aroma compounds, such as acetoin and diacetyl, from citrate. The first step in citrate utilization is its uptake by the cells. In L. diacetylactis, the citrate transport system is encoded by the citQRP operon. We have previously proposed that expression of citQRP operon is regulated at the post-transcriptional level. In this paper, we show that the cit mRNA is processed at a complex secondary structure in L. diacetylactis and Escherichia coli. This secondary structure includes the 5′-terminal two-thirds of citQ and the overlap between citQ and citR. Primer-extension analysis revealed that the major cleavage sites are located upstream of citR and within citQ. In an attempt to identify the enzyme(s) responsible for this cleavage, we have analyzed this processing in E.␣coli mutants deficient in endoribonucleases. A comparative analysis of cit mRNA degradation was performed in RNase E and RNase III mutants and in wild-type strains using Northern blot hybridization. This analysis revealed that the cit transcript is degraded into several breakdown products, which are significantly stabilized in the mutant lacking RNase III. Our results indicate that the complex secondary structure has a critical role in the control of the expression of cit mRNA. A model for processing is discussed.

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