Cloning of ribosomal RNA genes from an Indian isolate of Giardia lamblia and the use of intergenic nontranscribing spacer regions in the differentiation of Giardia from other enteric pathogens

The ribosomal RNA genes from an Indian isolate of Giardia lamblia have been cloned and characterized with respect to size, composition and copy number. Southern blotting and rDNA cloning of Giardia lamblia revealed that genes coding for ribosomal RNA (rRNA) are exceptionally small and are encoded within a 5-6 kb genome fragment repeat. The rDNA repeat unit of this isolate was found to be highly G-C rich like other human isolates and the physical map showed several Sinai sites. There are 132 copies of the rDNA repeat unit per cell in a head to tail arrangement. Two fragments corresponding to intergenic (0.2 kb and 0.3 kb) region and one (0.8 kb) containing both an intergenic region and a small part of the small subunit ribosomal RNA (SS rRNA) have been identified within the rDNA. These were used in heterogeneity studies of Giardia isolated from two geographic locations as well as in the analysis of cross reactivity with other enteric organisms. In Southern blots, all the three fragments were found to be highly specific for the differential diagnosis of Giardia spp. from the other enteric pathogens. These findings should help in developing a sensitive and more specific method for the diagnosis of giardiasis over currently available techniques.