Two adjacent genes encoding copper‐induced‐type metallothioneins in Tetrahymena thermophila may have resulted from a relatively recent gene duplication

Abstract

Tetrahymena thermophila is a valuable model organism for the study of metallothioneins, which play important roles in the metabolism of essential metal ions and in heavy‐metal detoxification. Starting from a T. thermophila expressed sequence tag (EST) originally annotated as a Zn–Cu–Cd metallothionein (GenBank Acc #BM394105), we determined the genomic sequence corresponding to this EST and its flaking region. The sequence contains two adjacent genes encoding metallothioneins of the copper‐induced type: the MTT2 gene (submitted to GenBank by Boldrin et al. as Cu‐inducible while this work was in progress) and a second, novel gene (MTT4), located ∼1.4 kb downstream from and with same orientation as MTT2. Our entire sequenced segment is contained in the T. thermophila whole‐genome sequence recently completed at TIGR. Both MTT4 and MTT2 are intronless and encode two nearly identical proteins (two differences out of 108 amino acids). The 5′ and 3′‐flanking regions of both the metallothionein genes show significant sequence similarity as well, suggesting recent gene duplication. All glutamines use the CAA codon, reminiscent of codon usage observed in highly expressed Tetrahymena genes. MTT4 and MTT2 map to a 1.5‐Mb macronuclear chromosome from the right arm of micronuclear chromosome 2.Supported by NIH grant RR02931. Sequence data were obtained from The Institute for Genomic Research Tetrahymena website, http://www.tigr.org/tdb/e2k1/ttg/