First, consider this abstract:

Annu Rev Cell Dev Biol. 2000;16:653-99. The Myc/Max/Mad network and the transcriptional control of cell behavior. Grandori C, Cowley SM, James LP, Eisenman RN.

The Myc/Max/Mad network comprises a group of transcription factors whose distinct interactions result in gene-specific transcriptional activation or repression. A great deal of research indicates that the functions of the network play roles in cell proliferation, differentiation, and death. In this review we focus on the Myc and Mad protein families and attempt to relate their biological functions to their transcriptional activities and gene targets. Both Myc and Mad, as well as the more recently described Mnt and Mga proteins, form heterodimers with Max, permitting binding to specific DNA sequences. These DNA-bound heterodimers recruit coactivator or corepressor complexes that generate alterations in chromatin structure, which in turn modulate transcription. Initial identification of target genes suggests that the network regulates genes involved in the cell cycle, growth, life span, and morphology. Because Myc and Mad proteins are expressed in response to diverse signaling pathways, the network can be viewed as a functional module which acts to convert environmental signals into specific gene-regulatory programs.

Keeping in mind that front-loading allows us to predict that major evolutionary transitions would be indebted to preadaptations, consider this recent finding:

Mol Biol Evol. 2011 May 13. Premetazoan Ancestry of the Myc-Max Network.Young S, Diolaiti D, Conacci-Sorrell M, Ruiz-Trillo I, Eisenman RN, King N.

The origin of metazoans required the evolution of mechanisms for maintaining differentiated cell types within a multicellular individual, in part through spatially differentiated patterns of gene transcription. The unicellular ancestor of metazoans was presumably capable of regulating gene expression temporally in response to changing environmental conditions, and spatial cell differentiation in metazoans may represent a co-option of pre-existing regulatory mechanisms. Myc is a critical regulator of cell growth, proliferation, and death that is found in all metazoans, but absent in other multicellular lineages, including fungi and plants. Homologs of Myc and its binding partner, Max, exist in two of the closest living relatives of animals, the choanoflagellate Monosiga brevicollis (Mb) and Capsaspora owczarzaki, a unicellular opisthokont that is closely related to metazoans and choanoflagellates. We find that Myc and Max from M. brevicollis heterodimerize and bind to both canonical and non-canonical E-boxes, the DNA binding sites through which metazoan Myc proteins act. Moreover, in M. brevicollis MbMyc protein can be detected in nuclear and flagellar regions. Like metazoan Max proteins, MbMax can form homodimers that bind to E-boxes. However cross-species dimerization between Mb and human Myc and Max proteins was not observed, suggesting that the binding interface has diverged. Our results reveal that the Myc/Max network arose before the divergence of the choanoflagellate and metazoan lineages. Furthermore, core features of metazoan Myc function, including heterodimerization with Max, binding to E-box sequences in DNA and localization to the nucleus, predate the origin of metazoans.


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