A novel family of mobile genetic elements is limited to the germline genome in Tetrahymena thermophila

Abstract

In the ciliated protozoan Tetrahymena thermophila, extensive DNA elimination is associated with differentiation of the somatic macronucleus from the germline micronucleus. This study describes the isolation and complete characterization of Tlr elements, a family of approximately 30 micronuclear DNA sequences that are efficiently eliminated from the developing macronucleus. The data indicate that Tlr elements are comprised of an approximately 22 kb internal region flanked by complex and variable termini. The Tlr internal region is highly conserved among family members and contains 15 open reading frames, some of which resemble genes encoded by transposons and viruses. The Tlr termini appear to be long inverted repeats consisting of (i) a variable region containing multiple direct repeats which differ in number and sequence from element to element and (ii) a conserved terminal 47 bp sequence. Taken together, these results suggest that Tlr elements comprise a novel family of mobile genetic elements that are confined to the Tetrahymena germline genome. Possible mechanisms of developmentally programmed Tlr elimination are discussed.

Self-synthesizing DNA transposons in eukaryotes

“…AAGF01001397, AAGF01001309, and AAGF01001471). INTs encoded by these sequences are Ϸ50% identical to INT encoded by Tlr transposable elements (18). Moreover, some copies of Tlr encode ATP (18), which is similar to the Polinton ATP.…”

“…INTs encoded by these sequences are Ϸ50% identical to INT encoded by Tlr transposable elements (18). Moreover, some copies of Tlr encode ATP (18), which is similar to the Polinton ATP. Tlr is characterized by 6-bp TSDs, Ͼ100-bp TIRs with the 5Ј-AGAGA terminus similar to that conserved in Polintons (ref.…”

Heterochromatin aggregation during DNA elimination in Tetrahymena is facilitated by a prion-like protein

“…DNA elimination in the exconjugants (progeny) of JUB6 MIC-KO was analyzed by using DNA fluorescent in situ hybridization (FISH) with probes complementary to the Tlr1 IESs, which show moderate levels of repetition (Wuitschick et al, 2002). At 36 hpm, Tlr1 IESs were detected in the new MICs, but not in the new MACs, in the exconjugants from wild-type cells (Fig.…”

Dynamic distributions of long double-stranded RNA in Tetrahymena during nuclear development and genome rearrangements

“…The development of new MACs triggers drastic genome rearrangements including fragmentation and amplification of the chromosomes and extensive internal deletion of one-third of the genome (Yao et al, 2014). The deleted sequences (referred to as internal eliminated sequences, IESs) largely comprise repetitive sequences and transposon-derived elements (Wuitschick et al, 2002;Fillingham et al, 2004). Removal of these elements from MACs can be considered an ultimate gene silencing mechanism.…”