Specific point mutations in Lactobacillus casei ATCC 27139 cause a phenotype switch from Lac− to Lac+

Abstract

Lactose metabolism is a changeable phenotype in strains ofLactobacillus casei. In this study, we found thatL. caseiATCC 27139 was unable to utilize lactose. However, when exposed to lactose as the sole carbon source, spontaneous Lac+clones could be obtained. A gene cluster (lacTEGF–galKETRM) involved in the metabolism of lactose and galactose inL. caseiATCC 27139 (Lac−) and its Lac+revertant (designated strain R1) was sequenced and characterized. We found that only one nucleotide, located in thelacTEGFpromoter (lacTp), of the twolac–galgene clusters was different. The protein sequence identity between thelac–galgene cluster and those reported previously for someL. casei(Lac+) strains was high; namely, 96–100 % identity was found and no premature stop codon was identified. A single point mutation located within thelacTppromoter region was also detected for each of the 41 other independently isolated Lac+revertants ofL. caseiATCC 27139. The revertants could be divided into six classes based on the positions of the point mutations detected. Primer extension experiments conducted on transcription fromlacTprevealed that thelacTppromoter of these six classes of Lac+revertants was functional, while that ofL. caseiATCC 27139 was not. Northern blotting experiments further confirmed that thelacTEGFoperon of strain R1 was induced by lactose but suppressed by glucose, whereas no blotting signal was ever detected forL. caseiATCC 27139. These results suggest that a single point mutation in thelacTppromoter was able to restore the transcription of a fully functionallacTEGFoperon and cause a phenotype switch from Lac−to Lac+forL.caseiATCC 27139.