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Lactobacillus reuteri LB 121 cells growing on sucrose synthesize large amounts of a glucan (d-glucose) and a fructan (d-fructose) with molecular masses of 3,500 and 150 kDa, respectively. Methylation studies and 13C or1H nuclear magnetic resonance analysis showed that the glucan has a unique structure consisting of terminal, 4-substituted, 6-substituted, and 4,6-disubstituted α-glucose in a molar ratio of 1.1:2.7:1.5:1.0. The fructan was identified as a (2→6)-β-d-fructofuranan or levan, the first example of levan synthesis by a Lactobacillus species. Strain LB 121 possesses glucansucrase and levansucrase enzymes that occur in a cell-associated and a cell-free state after growth on sucrose, raffinose, or maltose but remain cell associated during growth on glucose. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of sucrose culture supernatants, followed by staining of gels for polysaccharide synthesizing activity with sucrose as a substrate, revealed the presence of a single glucansucrase protein of 146 kDa. Growth of strain LB 121 in chemostat cultures resulted in rapid accumulation of spontaneous exopolysaccharide-negative mutants that had lost both glucansucrase and levansucrase (e.g., strain K-24). Mutants lacking all levansucrase activity specifically emerged following a pH shiftdown (e.g., strain 35-5). Strain 35-5 still possessed glucansucrase and synthesized wild-type glucan.

A regulatory region of the temperate Lactococcus lactis bacteriophage r1t chromosome has been cloned and characterized. It encompasses the two divergently oriented genes rro, encoding the phage repressor, and tec. Both genes, of which the transcription start sites have been mapped, are preceded by consensus -35 and -10 promoter sequences. The region contains three 21 bp direct repeats with internal dyad symmetry which probably act as operators. Two of these repeats partially overlap the two promoter sequences. The distant third repeat is located within the tec coding sequence. Gel mobility shift assays demonstrated that Rro specifically binds to this sequence. To study possible transcriptional regulation of the region, a lacZ translational fusion with an open reading frame following tec was constructed. Under conditions that favour the lysogenic life cycle of r1t, beta-galactosidase activity was very low. Expression of the lacZ fusion could be induced 70-fold by the addition of mitomycin C at a concentration which promotes the switch of r1t from the lysogenic to the lytic life cycle. In non-induced cells, promoter activity was repressed by Rro, as a frameshift mutation in rro resulted in constitutive expression of the lacZ gene fusion.

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