Lacticin Q (LnqQ) produced by Lactococcus lactis QU 5 is an unmodified linear bacteriocin, which is synthesized without an N-terminal leader peptide. In vitro synthesis and in vivo expression of LnqQ have revealed the intracellular toxicity of this leaderless peptide, as well as the necessity of a dedicated secretion and self-immunity system of producer cells. Further DNA sequencing and analysis have discovered 11 putative orf genes at the LnqQ locus. None of the orf genes showed similarities to any of the bacteriocin biosynthetic genes characterized to date; however, six orf genes (orf2q-7q), not including the structural gene (lnqQ), were highly conserved at the lacticin Z locus (orf2z-7z), which is a LnqQ homologue produced by L. lactis QU 14. ORF2q (ORF2z), the gene of which is located upstream of the structural gene, is a putative transcriptional regulator, whereas ORF6q and ORF7q (ORF6z and ORF7z) form a putative ATP-binding cassette transporter. The ORF3q-5q (ORF3z-5z) are all predicted to be membrane proteins with no clear functions. Co-expression of LnqQ and ORF3q-7q in a heterologous host allowed the extracellular production of LnqQ; additionally, the expression of ORF3q-7q rendered the host cells immune to LnqQ. This self-immunity was facilitated possibly by two means; firstly, by secreting the active LnqQ peptides, thus reducing the intracellular toxicity, and secondly, by protecting the host cells from extracellularly released LnqQ. This is the first report, to our knowledge, that describes intracellular toxicity of a leaderless bacteriocin and provides a rare example of biosynthetic genes that are required for bacteriocin secretion and immunity.
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