Lactococcus lactis is a nonpathogenic AT-rich gram-positive bacterium closely related to the genus Streptococcus and is the most commonly used cheese starter. It is also the best-characterized lactic acid bacterium. We sequenced the genome of the laboratory strain IL1403, using a novel two-step strategy that comprises diagnostic sequencing of the entire genome and a shotgun polishing step. The genome contains 2,365,589 base pairs and encodes 2310 proteins, including 293 protein-coding genes belonging to six prophages and 43 insertion sequence (IS) elements. Nonrandom distribution of IS elements indicates that the chromosome of the sequenced strain may be a product of recent recombination between two closely related genomes. A complete set of late competence genes is present, indicating the ability of L. lactis to undergo DNA transformation. Genomic sequence revealed new possibilities for fermentation pathways and for aerobic respiration. It also indicated a horizontal transfer of genetic information from Lactococcus to gram-negative enteric bacteria of Salmonella-Escherichia group. [The sequence data described in this paper has been submitted to the GenBank data library under accession no. AE005176.]

The Complete Genome Sequence of the Lactic Acid Bacterium Lactococcus lactis ssp. lactis IL1403

The kinetics, control, and efficiency of nisin-induced expression directed by the nisA promoter region were studied in Lactococcus lactis with transcriptional and translational fusions to the gusA reporter genes. In the nisin-producing L. lactis strain NZ9700, the specific beta-glucuronidase activity increased very rapidly after mid-exponential growth until the maximum level at the start of the stationary phase was reached. Expression of the gusA gene was also studied in L. lactis NZ9800, an NZ9700 derivative carrying a deletion in the structural nisA gene that abolishes nisin production, and in L. lactis NZ3900, an MG1363 derivative containing the regulatory nisRK genes integrated in the chromosome. In both strains, beta-glucuronidase activity was linearly dependent on the amount of nisin added to the medium. Without nisin, no beta-glucuronidase production was observed. To optimize translation initiation, an expression vector was constructed by fusing the gusA gene translationally to the start codon of the nisA gene. Use of the translational fusion vector yielded up to six times more beta-glucuronidase activity than the transcriptional fusion vector in these strains after induction by nisin. In this way, gene expression can be achieved in a dynamic range of more than 1,000-fold. The beta-glucuronidase activity was found to be up to 25-fold higher in extracts of strain NZ3900 than in extracts of strain NZ9800. This translational fusion vector was used for high-level production of aminopeptidase N, up to 47% of the total intracellular protein. These results clearly illustrate the potential of the nisin-inducible expression system for overproduction of desired proteins.

https://aem.asm.org/content/aem/62/10/3662.full.pdf

Controlled gene expression systems for Lactococcus lactis with the food-grade inducer nisin.

https://www.cghjournal.org/article/S1542-3565(06)00331-4/pdf

A Phase I Trial With Transgenic Bacteria Expressing Interleukin-10 in Crohn’s Disease

The nisin gene cluster nisABTCIPR of Lactococcus lactis, located on a 10-kbp DNA fragment of the nisin-sucrose transposon Tn5276, was characterized. This fragment was previously shown to direct nisin-A biosynthesis and to contain the nisP and nisR genes, encoding a nisin leader peptidase and a positive regulator, respectively [van der Meer, J. R., Polman, J., Beerthuyzen, M. M., Siezen, R. J., Kuipers, O. P. & de Vos, W. M. (1993) J. Bacteriol. 175, 2578–2588]. Further sequence analysis revealed the presence of four open-reading frames, nisB, nisT, nisC and nisI, downstream of the structural gene nisA. The nisT, nisC and nisI genes were subcloned and expressed individually in Escherichia coli, using the T7-RNA-polymerase system. This resulted in the production of radio-labelled proteins with sizes of 45 kDa (NisC) and 32 kDa (NisI). The nisT gene product was not detected, possibly because of protein instability. The deduced amino acid sequence of NisI contained a consensus Iipoprotein signal sequence, suggesting that this protein is a lipid-modified extracellular membrane-anchored protein. Expression of nisI in L. Iactis provided the cells with a significant level of protection against exogeneously added nisin, indicating that NisI plays a role in the immunity mechanism. In EDTA-treated E. coli cells, expression of nisI conferred up to a 170-fold increase in immunity against nisin A compared to controls. Moreover, a lactococcal strain deficient in nisin-A production, designated NZ9800, was created by gene replacement of nisA by a truncated nisA gene and was 10-fold less resistant to nisin A than the wild-type strain. A wild-type immunity level to nisin and production of nisin was obtained in strain NZ9800 harboring complementing nisA and nisZ plasmids. Transcription analyses of several L. IIactis strains indicated that an expression product of the nisA gene, together with NisR, is required for the activation of nisA transcription.

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Characterization of the nisin gene cluster nisABTCIPR of Lactococcus lactis. Requirement of expression of the nisA and nisI genes for development of immunity

https://pure.rug.nl/ws/files/14672157/1995JBiolChemKuipersO.pdf

Autoregulation of nisin biosynthesis in Lactococcus lactis by signal transduction.

The production and regeneration of bacterial protoplasts promoted the loss of three different plasmid-specified traits in Streptococcus lactis subsp. diacetylactis strains. The loss of five different plasmids, including small multicopy molecules, was readily detected in Streptococcus lactis 712 by screening lysates of random protoplast regenerants on agarose gels. In this strain sequential rounds of protoplast regeneration were used to produce a plasmid-free strain and derivatives carrying only single molecules from the plasmid complement. During these experiments a 33-megadalton plasmid, pLP712, was found to encode genes for lactose and protein utilization. Only this plasmid was required for normal growth and acid production in milk; the remaining four plasmids appeared to be cryptic. Lactose-defective derivatives of a strain carrying only pLP712 were readily isolated. Although these derivatives included instances of plasmid loss, deletions of pLP712 were frequently found. Many different deleted derivatives of pLP712, including some in which the lactose or protein utilization determinant or both were lost, were isolated. The molecular instability of pLP712 largely accounted for previous observations of plasmid complements in S. lactis 712 after lactose determinant curing or transfer by conjugation and transduction. Curing of cryptic molecules from multiple plasmid complements by protoplast regeneration may prove to be generally valuable in lactic streptococci and other gram-positive species.

Michaelj . Gasson

Papers

Plasmid complements of Streptococcus lactis NCDO 712 and other lactic streptococci after protoplast-induced curing.