Lactobacillus plantarum ccl gene is non-essential, arginine-repressed and codes for a conserved protein in Firmicutes

Among proteins specifically found in most gram-positive bacteria of the phylum Firmicutes, conserved proteins of the family pfam06177-DUF988-COG4708 are of unknown function. The citrulline cluster-linked (ccl) gene of Lactobacillus plantarum codes one such protein and is adjacent to the citrulline biosynthesis operon argCJBDF, a situation also found in Lactococcus lactis. This gene is well conserved among L. plantarum species, and 1 isolate out of 24 harbored two ccl copies. Northern hybridization with a ccl probe revealed two arginine-repressed transcripts with sizes corresponding to the predicted argCJBDF-ccl operon and the ccl gene alone. Transcription start sites of both transcripts were characterized. Four different 5' ends were mapped at the argF-ccl intergenic region, resulting from either regulated transcription initiation or maturation of the transcripts. Transcriptional ccl-gusA gene fusion confirmed the promoter activity of the argF-ccl intergenic region. Thus, the ccl gene is arginine-repressed and transcribed both monocistronically and polycistronically in the argCJBDF-ccl operon. The ccl gene is not essential in L. plantarum, because a ccl gene deletion was obtained in strain CCM 1904. Although no functions were found in the tested laboratory conditions, the Ccl-like proteins may play a role in environmental conditions of life.