Physical and genetic map of the Lactobacillus sakei 23K chromosome

The Lactobacillus sakei 23K chromosome was analysed by pulsed-field gel electrophoresis after digestion with the restriction enzymes Ascl, Notl and Sfil. The chromosome size was estimated to be 1845+/-80 kb. The use of I-Ceul, specific for rrn genes encoding 23S rRNAs, showed that seven rrn loci were present, on 40% of the chromosome. The seven rrn clusters were mapped and their orientation was determined, allowing the position of the replication origin to be estimated. Partial I-Ceul digestions were used to construct a backbone and the different restriction fragments obtained with Ascl, Notl and Sfil were assembled to a physical map by Southern hybridization. Eleven L. sakei gene clusters previously identified were mapped, as well as 25 new loci located randomly on the chromosome and 11 regions flanking the rrn gene clusters. A total of 47 clusters were thus mapped on L. sakei chromosome. The new loci were sequenced, allowing the identification of 73 complete or incomplete coding sequences. Among these 73 new genes of L. sakei, the function of 36 could be deduced from their similarity to known genes described in databases. However, 10 genes had no homologues, 10 encoded proteins similar to proteins of unknown function and 17 were similar to hypothetical proteins.