Innovation in gene regulation: the case of chromatin computation

Chromatin regulation is understood to be one of the fundamental modes of gene regulation in eukaryotic cells. We argue that the basic proteins that determine the chromatin architecture constitute an evolutionary ancient layer of transcriptional regulation common to all three domains of life. We explore phylogenetically, sources of innovation in chromatin regulation, focusing on protein domains related to chromatin structure and function, demonstrating a step-wise increase of complexity in chromatin regulation. Building upon the highly conserved use of variants of chromosomal architectural proteins to distinguish chromosomal states, Eukarya secondarily acquired mechanisms for "writing" chemical modifications onto chromatin that constitute persistent signals. The acquisition of reader domains enabled decoding of these complex, signal combinations and a decoupling of the signal from immediate biochemical effects. We show how the coupling of reading and writing, which is most prevalent in crown-group Eukarya, could have converted chromatin into a powerful computational device capable of storing and processing more information than pure cis-regulatory networks.