I think that perhaps a better, certainly an alternative, way of answering the question 'how did the first hox gene arrive?' is to look not at the higher level of regulatory signals such as Brahma but at its evolutionary precursors amongst the abundant homeobox containing proteins, especially as Brahma is only a drosophila relative of the SWI2/SNF2 containing proteins a family which itself covers an awful lot of ground.
A homeotic gene is a very vague functional description, only saying what the gene does and nothing about what it is. The homeobox genes however have very specific structural features which allow their DNA binding activity and their role as transcriptional regulators involved in a huge variety of areas from the aforementioned polarity/ patterning to cell cyle regulation.
Genes Dev. 2002 Dec 1;16(23):3034-45.
Pramila T, Miles S, GuhaThakurta D, Jemiolo D, Breeden LL. Conserved homeodomain proteins interact with MADS box protein Mcm1 to restrict ECB-dependent transcription to the M/G1 phase of the cell cycle.
Two homeodomain proteins, Yox1 and Yhp1, act as repressors at early cell cycle boxes (ECBs) to restrict their activity to the M/G1 phase of the cell cycle in budding yeast. These proteins bind to Mcm1 and to a typical homeodomain binding site. The expression of Yox1 is periodic and directly correlated with its binding to, and repression of, ECB activity. The absence of Yox1 and Yhp1 or the constitutive expression of Yox1 leads to the loss of cell-cycle regulation of ECB activity. Therefore, the cell-cycle-regulated expression of these repressors defines the interval of ECB-dependent transcription. Twenty-eight genes, including MCM2-7, CDC6, SWI4, CLN3, and a number of genes required during late M phase have been identified that are coordinately regulated by this pathway.
The real question is possibly that of how particular domains, such as the various DNA binding and regulatory domains, first evolved, this is always going to be highly speculative however due to the huge tracts of evolutionary time since those initial genes/proteins were extant. As soon as a few of these domains appeared then the possibilities for combining and domain swapping and consequently varied function increased enormously.
™ Version 4.2
