Micronucleus-specific histone H1 is required for micronuclear chromosome integrity in Tetrahymena thermophila
November
Micronucleus-specific histone H1 is required for micronuclear chromosome integrity in Tetrahymena thermophila
Juxia Qiao 0 1
Jing Xu 0 1
Tao Bo 0 1
Wei Wang 0 1
0 Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Biotechnology, Shanxi University , Taiyuan, Shanxi , China , 2 College of Life Science, Shanxi University , Taiyuan, Shanxi , China , 3 Institute of Evolution & Marine Biodiversity, Ocean University of China , Qingdao, Shandong , China
1 Editor: Douglas L. Chalker, Washington University in Saint Louis , UNITED STATES
Histone H1 molecules play a key role in establishing and maintaining higher order chromatin structures. They can bind to linker DNA entering and exiting the nucleosome and regulate transcriptional activity. Tetrahymena thermophila has two histone H1, namely, macronuclear histone H1 and micronuclear histone H1 (Mlh1). Mlh1 is specifically localized at micronuclei during growth and starvation stages. Moreover, Mlh1 is localized around micronuclei and forms a specific structure during the conjugation stage. It co-localizes partially with spindle apparatus during micronuclear meiosis. Analysis of MLH1 knock-out revealed that Mlh1 was required for the micronuclear integrity and development during conjugation stage. Overexpression of Mlh1 led to abnormal conjugation progression. RT-PCR analysis indicated that the expression level of HMGB3 increased in ΔMLH1 strains, while the expression level of MLH1 increased in ΔHMGB3 cells during conjugation. These results indicate that micronuclear integrity and sexual development require normal expression level of Mlh1 and that HmgB3 and Mlh1 may functionally compensate each other in regulating micronuclear structure in T. thermophila.
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Data Availability Statement: All relevant data are
within the paper and its Supporting Information
files.
Introduction
Chromatin in eukaryotic cells consists of DNA, histones, non-histones and RNA. The nucleo
some is the basic unit of chromatin with 146 bp of DNA wrapped around core histone octamer
consisting of two copies of evolutionarily conserved core histones, namely H2A, H2B, H3, and
H4 [
1
]. Histone H1 and non-histone chromosomal proteins bind to DNA between
nucleosomes [
2
]. Histone H1 is variable across organisms and mediates gene-specific regulation of
transcription as well as DNA-dependent processes [3±5]. H1 also provides a platform for
chromatin remodeling and efficient repair of DNA damage [
6
]. Multiple histone H1 family
members are present in different organisms. Eleven H1 variants have been found in mammals, five
in Xenopus laevis and eight in Caenorhabditis elegans, but only one linker histone H1 has been
identified in Physarum polycephalum and Drosophila melanogaster [
5, 7, 8
]. In higher
had no role in study design, data collection and
analysis, decision to publish, or preparation of the
manuscript.
eukaryotes, most H1 variants have similar structure, comprising primarily of an N-terminal
region, a C-terminal tail, and a central conserved globular region [
9
]. In Saccharomyces
cerevisiae, Hho1p contains two regions of sequence homology to the central globular domain of the
canonical histone H1 [
10
].
Tetrahymena has two different nuclei, a macronucleus (Mac) and a micronucleus (Mic).
Mac divides amitotically and is transcriptionally active, while Mic divides mitotically and is
transcriptionally inactive at vegetative stages. The macronuclear histone H1 is smaller, more
basic, and lacks a conserved globular domain [
11, 12
]. Functionally, macronuclear histone H1
is not essential for chromatin packaging, condensation and cellular viability [13]. However, it
can regulate the expression of specific genes [
14
]. Micronucleus-specific histone H1 (Mlh1) is
different from macronuclear H1 and H1 from other organisms [
15
]. Mlh1 is larger and is
proteolytically processed into different components (α, β, γ, and δ) [
16
]. Both β and γ resemble Hl
histones but lack a globular domain [
13
]. δ contains two high mobility group (HMG) boxes,
while α is simply the uncleaved fusion of δ and γ [
16
].
Knocking out MLH1 does not affect Tetrahymena viability and cellular survival during veg
etative growth stage, but causes enlarged Mic. Macronuclear histone H1 knock-out cells have
enlarged Mac and normal-sized Mic [
13
]. However, the function of Mlh1 during sexual
development is unclear. In the present study, we focused on the localization and function of Mlh1
during sexual development. We found that Mlh1 maintained micronuclear chromosome
integrity and thus played an important role in Tetrahymena conjugation.
Materials and methods
Tetrahymena strains and culture conditions
The B2086 and CU428 T. thermophila strains were provided by P. J. Bruns (Cornell University,
now available at the National Tetrahymena Stock Center, http://tetrahymena.vet.cornell.edu/
index.html). Cells were cultured in super proteose peptone (...truncated)