Characterization of a mobile clpL gene from Lactobacillus rhamnosus

Two genes encoding ClpL ATPase proteins were identified in a probiotic Lactobacillus rhamnosus strain, E-97800. Sequence analyses revealed that the genes, designated clpL1 and clpL2, share 80% identity. The clpL2 gene showed the highest degree of identity (98.5%) to a clpL gene from Lactobacillus plantarum WCFSI, while it was not detected in three other L. rhamnosus strains studied. According to Northern analyses, the expression of clpL1 and the clpL2 were induced during heat shock by > 20- and 3-fold, respectively. The functional promoter regions were determined by primer extension analyses, and the clpL1 promoter was found to be overlapped by an inverted repeat structure identical to the conserved CIRCE element, indicating that clpL1 belongs to the HrcA regulon in L. rhamnosus. No consensus binding sites for HrcA or CtsR could be identified in the clpL2 promoter region. Interestingly, the clpL2 gene was found to be surrounded by truncated transposase genes and flanked by inverted repeat structures nearly identical to the terminal repeats of the ISLpl1 from L. plantarum HN38. Furthermore, clpL2 was shown to be mobilized during prolonged cultivation at elevated temperature. The presence of a gene almost identical to clpL2 in L. plantarum and its absence in other L. rhamnosus strains suggest that the L. rhamnosus E-97800 has acquired the clpL2 gene via horizontal transfer. No change in the stress tolerance of the ClpL2-deficient derivative of E-97800 compared to the parental strain was observed.