NUCLEOTIDE-SEQUENCE ORGANIZATION AND ANALYSIS OF THE NUCLEAR RIBOSOMAL DNA CIRCLE OF THE PROTOZOAN PARASITE ENTAMOEBA-HISTOLYTICA

We have sequenced the extrachromosomal ribosomal DNA (rDNA) circle of the human protozoan parasite Entamoeba histolytica HM-1:IMSS and prese nt here the complete sequence organisation of the 24.5-kb molecule. Ea ch circle contains two 5.9-kb rDNA transcription units organised as in verted repeats. The regions downstream (3543 bp) and upstream (9216 bp ) of the rDNAs contain various families of short tandem repeats. Some of the upstream repeats share extensive sequence homology with the dow nstream repeats. In addition to the rDNAs themselves, the rDNA circle appears to code for only one other transcript which is 0.7 kb in size as seen in Northern blots. From DNA sequence analysis, no open reading frame could be assigned to the transcript. Extrachromosomal rDNA circ les also exist in other E. histolytica strains. Restriction enzyme map s of rDNA circles were constructed from E. histolytica strains 200:NIH , HK-9 and Rahman; and Entamoeba moshkovskii strain Laredo. Striking d ifferences were observed in the organisation of some of them, e.g. the HK-9, Rahman and Laredo circles contained only one rDNA unit and lack ed the 0.7-kb transcript sequence. The short repeat sequences upstream and downstream of rDNAs were present in HK-9 and Rahman but absent in Laredo. Circles with one rDNA unit may be derived from those with two units by homologous recombination at direct repeat sequences located upstream and downstream of the two rDNAs.