Unusual Gene Organization in the Protozoan Parasite <i>Entamoeba histolytica</i>

We have analyzed three independent genomic loci of the protozoan parasite Entamoeba histolytica that contain coding regions for the iron-containing superoxide dismutase, the pore-forming peptide, and the galactose-inhibitable lectin. All of the three structural genes were found to be closely linked unidirectionally to other coding sequences. The intergenic regions did not exceed 1,350 nucleotides. Nuclear run-on data demonstrated that at least the galactose-inhibitable lectin gene is transcribed in a monocistronic fashion. Comparison of the genomic sequences described here with several others reported previously for E. histolytica revealed a number of invariable peculiarities for the gene organization of this parasite: (i) Coding sequences are not interrupted by introns; (ii) 5' untranslated regions are rather short and transcription starts at the consensus sequences ATTCA or ATCA; (iii) an unusual TATA-motif is located about 30 nucleotides upstream of the start of transcription and comprises the sequence TATTTAAA, which reveals protein binding activity as determined by gel retardation assays; (iv) the conserved pentanucleotide motif TAA/TTT is found within the relatively short 3' untranslated regions and functions putatively as the transcription termination signal; and (v) a stretch of up to 12 pyrmidine residues is located at the end of transcribed sequences.