My reason for favouring the ‘enol’ over the ‘keto' configuration within the conformation of the major helix is simply down to the fact that it places the oxygen attached to C4 of Thymine within the centre of the primary helix and hence amongst the NH2’s that make up the secondary helix within the primary. This strikes me as potentially awkward.  That said, it may well be that by placing this oxygen within the NH2 helical sequence facilitates some function in relation to amino acid synthesis.