SALMONELLA-TYPHI CONTAINS IDENTICAL INTERVENING SEQUENCES IN ALL 7 RRL GENES

Salmonella typhi Ty2 rrl genes contain intervening sequences (IVSs) in helix-25 but not in helix-45 on the basis of observed 23S rRNA fragme ntation caused by IVS excision. We have confirmed this and shown all s even IVSs to be identical by isolating genomic DNA fragments containin g each of the seven rrl genes from S.typhi Ty2 by use of pulsed-field gel electrophoresis; each rrl gene was amplified by PCR in the helix-2 5 and helix-45 regions and cycle sequenced. Thirty independent wild-ty pe S. typhi strains, tested by genomic PCR and DraI restriction, also have seven rrl genes with helix-25 IVSs and no helix-45 IVSs. We propo se that TVS homogeneity in S. typhi occurs because gene conversion dri ves TVS sequence maintenance and because adaptation to human hosts res ults in limited clonal diversity.