The complete genome sequence of Lactobacillus bulgaricus reveals extensive and ongoing reductive evolution.

van de Guchte M, Penaud S, Grimaldi C, Barbe V, Bryson K, Nicolas P, Robert C,
Oztas S, Mangenot S, Couloux A, Loux V, Dervyn R, Bossy R, Bolotin A, Batto JM,
Walunas T, Gibrat JF, Bessières P, Weissenbach J, Ehrlich SD, Maguin E.

Lactobacillus delbrueckii ssp. bulgaricus (L. bulgaricus) is a representative of
the group of lactic acid-producing bacteria, mainly known for its worldwide
application in yogurt production. The genome sequence of this bacterium has been
determined and shows the signs of ongoing specialization, with a substantial
number of pseudogenes and incomplete metabolic pathways and relatively few
regulatory functions. Several unique features of the L. bulgaricus genome support
the hypothesis that the genome is in a phase of rapid evolution. (i)
Exceptionally high numbers of rRNA and tRNA genes with regard to genome size may
indicate that the L. bulgaricus genome has known a recent phase of important size
reduction, in agreement with the observed high frequency of gene inactivation and
elimination; (ii) a much higher GC content at codon position 3 than expected on
the basis of the overall GC content suggests that the composition of the genome
is evolving toward a higher GC content; and (iii) the presence of a 47.5-kbp
inverted repeat in the replication termination region, an extremely rare feature
in bacterial genomes, may be interpreted as a transient stage in genome
evolution. The results indicate the adaptation of L. bulgaricus from a
plant-associated habitat to the stable protein and lactose-rich milk environment
through the loss of superfluous functions and protocooperation with Streptococcus
thermophilus.

Proc Natl Acad Sci U S A. 2006 Jun 13;103(24):9274-9. Epub 2006 Jun 5.