Molecular characterisation of the dnaK operon of Lactobacillus sakei LTH681

The use of lactobacilli as starter organisms in food fermentation processes requires thorough knowledge of their reaction to the multitude of ecological factors including their response to stress. We have characterised the dnaK gene region of Lactobacillus sakei LTH681. Two chromosomal EcoRI fragments of 2.5 and 4.0 kb were identified using a homologous dnaK probe generated by PCR. The sequence analysis of the cloned fragments showed that the dnaK gene region consists of four heat shock genes with the organisation hrcA-grpE-dnaK-dnaJ. Comparison of the deduced amino acid sequences revealed high similarity to the corresponding heat shock proteins of Gram-positive bacteria. An upstream located orfY was found which exhibited substantial similarity (41.5%) to the chloramphenicol acetyltransferase of Enterobacter aerogenes. Northern hybridisation analysis revealed that the transcription of the genes is induced by heat shock (42 degrees C) as well as salt (6%) or ethanol (10%) stress. Several transcripts were detected including a polycistronic mRNA of 4.9 kb which represents the transcript of the complete dnaK gene region indicating a tetracistronic organisation of the dnaK operon. The other RNA fragments were identified as shorter transcripts (3.7 and 1.3 kb) or cleavage products of the polycistronic mRNAs. The transcription start sites of the dnaK operon were determined under inducing and non-inducing conditions. The site varied with the applied stress condition. A regulatory CIRCE element was identified located between the transcription and translation start site. The promoter region including CIRCE was transcriptionally fused to the beta-glucuronidase reporter gene gusA and expressed in L. sakei LTH681. The kinetics of transcriptional induction of gusA by heat shocking were identical to those of the dnaK operon confirming the involvement of the CIRCE element in regulation of gene expression.