Diverse sequences within Tlr elements target programmed DNA elimination in Tetrahymena thermophila

Area of Science:

• Molecular Biology
• Genetics
• Developmental Biology

Background:

• Tetrahymena thermophila possesses a germ line micronuclear genome and a somatic macronuclear genome.
• Mobile genetic elements, such as Tlr elements, are confined to the germ line genome.
• Germ line-limited sequences are eliminated from the developing somatic macronucleus, with flanking sequences known to contain cis-acting elimination signals.

Purpose of the Study:

• To investigate whether sequences within mobile genetic elements themselves play a regulatory role in their own elimination.
• To identify internal cis-acting sequences required for the programmed DNA elimination of Tlr elements.

Main Methods:

• Utilized an in vivo DNA rearrangement assay in Tetrahymena thermophila.
• Assayed the ability of different regions of Tlr elements to stimulate DNA elimination when placed in a new genomic context.
• Replaced Tlr element DNA with macronuclear or foreign DNA to assess the necessity of Tlr sequences for elimination.

Main Results:

• Multiple, nonoverlapping internal regions of Tlr elements were found to be sufficient for stimulating developmentally regulated DNA elimination.
• Replacing Tlr element DNA with non-Tlr DNA abolished elimination activity, confirming the role of Tlr sequences.
• Surprisingly, Tlr DNA was efficiently eliminated even when flanking sequences were replaced with non-rearranging genomic regions.

Conclusions:

• Diverse sequences dispersed throughout Tlr DNA contain cis-acting signals that target these elements for programmed elimination.
• Specific flanking sequences are not strictly required for the elimination of Tlr element DNA, challenging previous assumptions.
• This study reveals novel regulatory mechanisms for programmed DNA elimination involving sequences within the mobile elements themselves.