Isochores and tissue-specificity.

Area of Science:

• Genomics
• Molecular Biology
• Evolutionary Biology

Background:

• Gene expression patterns and GC content are key genomic features.
• Housekeeping genes are ubiquitously expressed, while tissue-specific genes are restricted to certain cell types.
• GC content variation influences genome structure and evolution.

Purpose of the Study:

• To investigate the relationship between gene GC content and expression patterns in mammalian genomes (human and mouse).
• To compare GC content distributions of housekeeping versus tissue-specific genes.
• To evaluate the biased gene conversion (BGC) hypothesis in light of observed GC content variations.

Main Methods:

• Comparative analysis of gene GC content across different expression categories (housekeeping, tissue-specific) in human and mouse genomes.
• Statistical analysis of GC content distribution and correlation with expression levels.
• Examination of GC content in genes specific to germ-line and non-germ-line tissues.

Main Results:

• Housekeeping genes exhibit slightly higher average GC content than tissue-specific genes, concentrating in the upper GC range.
• Human tissue-specific genes show bimodal GC distribution (GC-poor and GC-rich), with strictly tissue-specific genes favoring the GC-poor region.
• Genes in certain tissues (e.g., central nervous system) have higher GC content than housekeeping genes, contradicting BGC.
• Germ-line specific genes (ovary, testes) show low average GC content, also contradicting BGC.
• A weak positive correlation exists between gene GC content and expression level.
• Mouse genomes show a higher fraction of ubiquitously expressed genes than human genomes, suggesting less molecular differentiation.

Conclusions:

• The GC content of mammalian genes is linked to their expression patterns, but not solely explained by biased gene conversion.
• Tissue-specific gene expression and GC content distributions vary between species, potentially reflecting differences in genome structure and tissue differentiation.
• The findings suggest complex evolutionary pressures shaping gene GC content beyond simple gene conversion mechanisms.