A micronucleus-specific sequence exists in the 5′-upstream region of calmodulin gene in Tetrahymena thermophila

Abstract

Tetrahymena thermophila possesses a transcriptionally inactive mlcronucleus and an active macronucleus. Both nuclei are developed from micronucleus-derived germ nuclei during conjugation. Extensive DNA rearrangement and transcrlptlonal activation are known to be involved In macronuclear development, but little has been known about these processes in a particular functional gene. Therefore the micro- and macronuclear genomic DNAs for calmodulin gene were analyzed. A 1,384 bp micronucleus-specific sequence located about 3.5 kb upstream of calmodulin gene has been found, suggesting DNA rearrangement during macronuclear development. The micronucleus-specific sequence had 85% A + T, no extensive ORF, ATT As at both ends, and two palindromlc structures just outside of both ends. Interestingly, the micronucleus-specific sequence included a T-rlch tract, T16CT5, in the middle, and a nearly complementary A-rich tract, A5TA10GA5, existed 7 bp upstream from the initiation codon. In addition, there was a 20 bp repetitive sequence TAAT(TAAC)4 about 100 bp upstream of the micronucleus-specific sequence and also In the promoter region of calmodulin gene. Although the functional significance of the micronucleus-specific sequence remains unclear, T16CT5 and TAAT(TAAC)4 elements might exert an influence on transcription of the calmodulin gene. Stringent Southern hybridization revealed that this micronucleus-specific sequence or very similar sequence(s) were abundant in the Tetrahymena mlcronuclear genome. © 1993 IRL Press at Oxford University Press.