Structure of a frequently rearranged rRNA-encoding chromosome in Giardia lamblia

Abstract

It has been shown previously that the rRNA encoding chromosomes in Giardia lamblia undergo frequent rearrangements with an estimated rate of ∼1% per cell per division (Le Blancq etal., 1992, Nucleic Acids Res., 17, 4539-4545). Following these observations, we searched for highly recombinogenlc regions in one of the requently rearranged rRNA encoding chromosomes, that is chromosome 1, a small, 1.1 Mb chromosome. Chromosome 1 undergoes frequent rearrangements that result in size variation of 5-20%. We analyzed the structure of chromosome 1 in clonal lineages from the WB strain. The two ends of chromosome 1 comprise telomere repeat [TAGGG] arrays joined to a truncated rRNA gene and a sequence referred to as '4e', respectively. Comparison of the structure of four polymorphic versions of chromosome 1, resulting from independent rearrangement events in four cloned lines, located a single polymorphic region to the variable rONA-telomere domain. Chromosome 1 is organized into two domains: a core region spanning -850 kb that does not exhibit size heterogeneity among different chromosome 1 and a variable region that spans 185-450 kb and includes the telomeric rRNA genes, referred to as the variable rDNA-telomere domain. The core region contains a conserved region, spanning ∼550 kb adjacent to the telomeric 4e sequence, which is only present in the 4e containing chromosomes and a 300 kb region of repetitive sequences that are also components of other chromosomes as well. Changes in the number of rDNA repeats accounted for some, but not all, of the size variation. Since there are four chromosomes that share the core region of chromosome 1, we suggest that the genome is tetraploid for this chromosome. © 1995 Oxford University Press.