We have isolated and sequenced a 525 bp ubiquitin cDNA (Acantub1) from the soil amoeba Acanthamoeba castellanii. The deduced amino acid (aa) sequence reveals that the Acantub1 gene product is a fusion protein consisting of 128 aa (76 aa ubiquitin and 52 aa 60S ribosomal protein). The Acantub1 protein sequence has unique substitutions at aa 28 (glutamine for alanine) and 121 (tryptophan for leucine) compared to the 8 highest homologous sequences in the databank. A putative 'zinc finger' nucleic acid-binding domain was located in the 52 aa ribosomal protein. Northern analyses reveal that the Acantub1 gene is regulated during development.