THE LAC OPERON OF LACTOBACILLUS-CASEI CONTAINS LACT, A GENE CODING FOR A PROTEIN OF THE BGLG FAMILY OF TRANSCRIPTIONAL ANTITERMINATORS

The 5' region of the lac operon of Lactobacillus casei has been invest igated. An open reading frame of 293 codons, designated lacT, was iden tified upstream of lacE. The gene product encoded by lacT is related t o the family of transcriptional antiterminator proteins, which include s BgIG from Escherichia coli, ArbG from Erwinia chrysanthemi, SacT, Sa cY, and LicT from Bacillus subtilis, and BgIR from Lactococcus lactis, Amino acid sequence identities range from 35 to 24%, while similariti es range from 56 to 47%, The transcriptional start site of the lac ope ron was identified upstream of lacT, The corresponding mRNA would cont ain in the 5' region a sequence with high similarity to the consensus RNA binding site of transcriptional antiterminators overlapping a sequ ence capable of folding into a structure that resembles a rho-independ ent terminator, LacT was shown to be active as an antiterminator in a B. subtilis test system using the sacB target sequence, lacT directly precedes lacEGF, the genes coding for enzyme IICB, phospho-beta-galact osidase, and enzyme IIA, and these genes are followed by a sequence th at appears to encode a second rho-independent transcription terminator -like structure. Northern hybridizations with probes against lacT, lac E, and lacF revealed transcripts of similar sizes for the lac mRNAs of several L. casei strains. Since the length of the the mRNA is just su fficient to contain lacTEGF, we conclude that the lac operon of L. cas ei does not contain the genes of the accessory tagatose-6-phosphate pa thway as occurs in the lac operons of Lactococcus lactis, Streptococcu s mutans, or Staphylococcus aureus.