A 3 NUCLEOTIDE SIGNATURE SEQUENCE IN SMALL-SUBUNIT RIBOSOMAL-RNA DIVIDES HUMAN GIARDIA IN 2 DIFFERENT GENOTYPES

The nucleotide sequence of the 16S rRNA gene, part of the 23S rRNA gen e and the spacer DNA region was determined for Giardia duodenalis, obt ained from humans in The Netherlands (AMC-4) and Washington State (CM) . These rDNA sequences differ from other G. duodenalis isolates (Portl and-1 and BRIS/83/HEPU/106) both of which have virtually identical rDN A sequences. The most characteristic feature was found close to the 5' end of the 16S rRNA. The Portland-1 - Bris/83/HEPU/106 type has GCG i n position 22-24, while AMC-4 and CM have AUC in this position. These two sequences, present in an otherwise conserved region of the 16S rRN A, are ''signature'' sequences, which divide Giardia isolates into two different groups.