GENOMIC ORGANIZATION OF LACTIC-ACID BACTERIA

Current knowledge of the genomes of the lactic acid bacteria, Lactococ cus lactis and Streptococcus thermophilus, and members of the genera L actobacillus, Leuconostoc, Pediococcus and Carnobacterium, is reviewed . The genomes contain a chromosome within the size range of 1.8 to 3.4 Mbp. Plasmids are common in Lactococcus lactis (most strains carry 4- 7 different plasmids), some of the lactobacilli and pediococci, but th ey are not frequently present in S. thermophilus, Lactobacillus delbru eckii subsp. bulgaricus or the intestinal lactobacilli. Five IS elemen ts have been found in L. lactis and most strains carry multiple copies of at least two of them; some strains also carry a 68-kbp conjugative transposon. IS elements have been found in the genera Lactobacillus a nd Leuconostoc, but not in S. thermophilus. Prophages are also a norma l component of the L. lactis genome and lysogeny is common in the lact obacilli, however it appears to be rare in S, thermophilus. Physical a nd genetic maps for two L. lactis subsp. lactis strains, two L. lactis subsp. cremoris strains and S. thermophilus A054 have been constructe d and each reveals the presence of six rrn operons clustered in less t han 40% of the chromosome. The L. lactis subsp. cremoris MG1363 map co ntains 115 genetic loci and the S. thermophilus map has 35. The maps i ndicate significant plasticity in the L. lactis subsp. cremoris chromo some in the form of a number of inversions and translocations. The cau se(s) of these rearrangements is (are) not known. A number of potentia lly powerful genetic tools designed to analyse the L. lactis genome ha ve been constructed in recent years. These tools enable gene inactivat ion, gene replacement and gene recovery experiments to be readily carr ied out with this organism, and potentially with other lactic acid bac teria and Gram-positive bacteria. Integration vectors based on tempera te phage attB sites and the random insertion of IS elements have also been developed for L. lactis and the intestinal lactobacilli. In addit ion, a L. lactis sex factor that mobilizes the chromosome in a manner reminiscent to that seen with Escherichia coli Hfr strains has been di scovered and characterized. With the availability of this new technolo gy, research into the genome of the lactic acid bacteria is poised to undertake a period of extremely rapid information accrual.