Histone deletion mutants challenge the molecular clock hypothesis.

Area of Science:

• Molecular Biology
• Evolutionary Biology
• Genetics

Background:

• The molecular clock hypothesis posits that protein sequence evolution rates correlate with functional constraints.
• Core histone proteins are known for their high conservation across eukaryotic species.
• Understanding the functional importance of different protein regions is crucial for evolutionary studies.

Purpose of the Study:

• To investigate the relationship between sequence conservation and functional indispensability in core histone proteins.
• To test the assumptions of the molecular clock hypothesis in the context of histone evolution.
• To identify regions of core histones that are dispensable for cellular function.

Main Methods:

• Comparative sequence analysis of core histone proteins across diverse eukaryotic taxa.
• Functional assays in yeast (Saccharomyces cerevisiae) to assess the impact of large-scale deletions in histone proteins on growth.
• Site-directed mutagenesis to probe the essentiality of specific histone residues and regions.

Main Results:

• Core histone sequences exhibit high conservation, supporting their fundamental role in eukaryotes.
• Despite overall conservation, extensive regions within core histone proteins were found to be dispensable for yeast viability and growth.
• This suggests that functional constraints, as predicted by the molecular clock hypothesis, may not apply uniformly across all parts of these proteins.

Conclusions:

• The high conservation of core histone sequences does not necessarily imply that all regions are functionally critical.
• A significant portion of core histone proteins can tolerate mutations or deletions without compromising essential cellular functions like growth.
• These findings challenge a strict interpretation of the molecular clock hypothesis, highlighting the complexity of protein evolution and functional constraints.