AguR, a Transmembrane Transcription Activator of the Putrescine Biosynthesis Operon in Lactococcus lactis, Acts in Response to the Agmatine Concentration

Abstract

bDairy industry fermentative processes mostly use Lactococcus lactis as a starter. However, some dairy L. lactis strains produce putrescine, a biogenic amine that raises food safety and spoilage concerns, via the agmatine deiminase (AGDI) pathway. The enzymatic activities responsible for putrescine biosynthesis in this bacterium are encoded by the AGDI gene cluster. The role of the catabolic genes aguB, aguD, aguA, and aguC has been studied, but knowledge regarding the role of aguR (the first gene in the cluster) remains limited. In the present work, aguR was found to be a very low level constitutively expressed gene that is essential for putrescine biosynthesis and is transcribed independently of the polycistronic mRNA encoding the catabolic genes (aguBDAC). In response to agmatine, AguR acts as a transcriptional activator of the aguB promoter (P aguB ), which drives the transcription of the aguBDAC operon. Inverted sequences required for P aguB activity were identified by deletion analysis. Further work indicated that AguR is a transmembrane protein which might function as a one-component signal transduction system that senses the agmatine concentration of the medium and, accordingly, regulates the transcription of the aguBDAC operon through a C-terminal cytoplasmic DNA-binding domain typically found in LuxR-like proteins. Lactococcus lactis is the lactic acid bacterium (LAB) most widely used as a primary starter in the dairy industry, especially in cheese manufacturing. Despite its qualified presumption of safety (QPS) status (awarded by the European Food and Safety Authority [EFSA]) and its generally regarded as safe (GRAS) status (awarded by the Food and Drug Administration [FDA]), some L. lactis strains possess enzymatic activities that produce undesirable flavors associated with food spoilage (1). Some even produce toxic compounds, such as the biogenic amine (BA) putrescine (2). Putrescine, together with histamine and tyramine, is one of the BAs in fermented dairy products most frequently encountered at potentially unsafe levels (3-5). It has a synergistic effect on the toxicity of other BAs and can also react with nitrite to form carcinogenic nitrosamines (4, 6). In addition, the metabolism of putrescine and of its derivatives (the polyamines spermine and spermidine) plays an important role in the promotion of colorectal tumorigenesis via effects on cell proliferation and migration (7-10).A number of putrescine-producing L. lactis strains of the subspecies L. lactis subsp. lactis and L. lactis subsp. cremoris isolated from artisanal cheeses have been shown to have a functional agmatine deiminase (AGDI) pathway. This catabolizes agmatine (a decarboxylated derivative of arginine) (11) into putrescine, yielding one molecule of ATP, one molecule of CO 2 , and two ammonium ions (2). The AGDI pathway increases the growth of L. lactis and causes the alkalinization of the culture medium, although it does not seem to be an acid stress resistance mechanism (12). The AGDI cluster of L. lactis is composed of five ge...

CerR, a Single-Domain Regulatory Protein of the LuxR Family, Promotes Cerecidin Production and Immunity in Bacillus cereus

“…4). The binding sites of regulators of the LuxR type, such as those of LuxR (24), PimM (a positive regulator of pimaricin biosynthesis) (25), and AguR (a transmembrane transcription activator of the putrescine biosynthesis operon in Lactococcus lactis), usually display dyad symmetry (26). Comparing these binding sites with the CerR binding box (GSAATWTTSC [S ϭ C or G; W ϭ A or T]), we found that the inverted repeated sequence of CerR shows an 8-bp match with PimM binding sites (TAGGGAATTCCCPA) but has no sequence similarity with those of the other two transcription regulators.…”

Biogenic Amines in Dairy Products: Origin, Incidence, and Control Means

“…Likewise, a recent in silico structure analysis predicted the AguR of a L. lactis subsp . cremoris cheese isolate to be a transmembrane protein with a typical LuxR_C‐like HTH DNA‐binding cytoplasmic C‐terminal motif (Linares and others ). This predicted AguR differs from that of Str.…”

“…mutans AguR which is cytoplasmic. Further characterization of the lactococcal AgDI cluster showed that aguR gene is expressed constitutively at low levels into AguR protein which functions as a 1‐component transduction system (Linares and others ). These authors suggested that the lactococcal AguR would play a pivotal role in both sensing the presence of agmatine in the extracellular medium and interacting with the aguB promoter (P aguB ) to activate the transcription of the aguBDAC operon into polycistronic mRNA to be translated into the 4 catabolic genes of the AgDI system.…”

Putrescine biosynthesis in Lactococcus lactis is transcriptionally activated at acidic pH and counteracts acidification of the cytosol

“…The aguR gene is transcribed as a monocistronic mRNA from its own promoter P aguR , independently of the aguBDAC genes. These latter genes are co-transcribed in a single polycistronic mRNA from the promoter of the first gene of the operon (P aguB ) (Linares et al, 2015). AguR is a transmembrane protein that functions as a one-component signal transduction system: it senses the agmatine concentration of the medium and regulates the transcription of the aguBDAC operon accordingly via a C-terminal cytoplasmic DNAbinding domain (Linares et al, 2015).…”