Figure 3.

Inactivating mutations in two operons illustrate the ongoing process of gene decay. The leu operon and the ilv operon, which work in concert, accumulated inactivating mutations in the genome of Francisella tularensis subspecies tularensis (F.t.t.) Schu S4 and F tularensis subspecies holarctica (F.t.h.) live vaccine strain (LVS). The ISFtu1 element that disrupted leuA and the ISFtu1 integrated upstream of leuB share the same bordering sequences in both genomes. The inactivating mutation in leuB is the same in both genomes as well. Therefore, these events are believed to have taken place in the leu operon before divergence into two subspecies. The other mutations in the regions of the leu operon and the ilv operon are of different origins in the two genomes, indicating that these mutations took place after the subspeciation.