Chromosomal constraints in Gram-positive bacteria revealed by artificial inversions.

We used artificial chromosome inversions to investigate the chromosomal constraints that preserve genome organization in the Gram-positive bacterium Lactococcus lactis. Large inversions, 80-1260 kb in length, disturbing the symmetry of the origin and terminus of the replication axis to various extents, were constructed using the site-specific Cre-loxP recombination system. These inversions were all mechanistically feasible and fell into various classes according to stability and effect on cell fitness. The L. lactis chromosome supports only to some extent unbalance in length of its replication arms. The location of detrimental inversions allowed identification of two constrained chromosomal regions: a large domain covering one fifth of the genome that encompasses the origin of replication (Ori domain), and a smaller domain located at the opposite of the chromosome (Ter domain).