Six putative two-component regulatory systems isolated from Lactococcus lactis subsp. cremoris MG1363 The GenBank accession numbers for the sequences of the six 2CSs and surrounding ORFs determined in this work are AF172649, AF176556, AF178425, AF172650, AF176557 and AF176809 for systems A–F, respectively.

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Abstract
The genetic elements specifying six putative two-component regulatory systems (2CSs) were identified on the chromosome of Lactococcus lactis MG1363. These 2CSs appear to represent distinct loci, each containing a histidine kinase and response-regulator-encoding gene pair. Transcriptional analysis of the six 2CSs was performed either by generating transcriptional fusions to a reporter gene or by primer extension. Two of the systems appeared to be expressed constitutively at a high level, whilst the remaining four exhibited growth-phase-dependent expression. Insertional mutagenesis established that the two constitutively expressed 2CSs are necessary for normal cell growth and/or survival. Mutational analysis of the remaining four systems revealed that they are implicated in susceptibility to extreme pH, osmotic or oxidative conditions, or the regulation of phosphatase activity in L. lactis.