• Which disciplines publish most in Nature?
  The % of papers in different fields over recent years:
  

Biochem, Genetics and Cell Biology together account for about half of total papers published in Nature. Unfortunately, there is no obvious way of separating simplifiers/constructors without reading all the papers. The fraction of physical science/engineering papers is about 38%. For comparison, the fraction of science academics in physical/eng disciplines to bio/medical is about 50% (see Ch2 number of academics).
This pecking order of which disciplines get the papers in the highest impact journals is broadly similar between Science and Nature, see below.
(the disciplines in the graph at the top of this page are ranked based on number of papers, both for Science and Nature, and then the correlation is plotted.)
We can also plot the % of each discipline correlated between these two journals:
The correlation is very systematic, but does not reveal any systematic bias between disciplines. Note that the % of papers in fields in Nature/Science is very different from the number of academics in these disciplines (at least from the UK RAE2008).

• In the UK, the number of papers published by different fields in 2010 looks likeso that around 35% are from clinical medicine, 15% biomedical, 6% biology, 10% are engineering, 9% physics, 7.5% chemistry and so on. This can be compared to the number of UK academics generating them (see numbers), so 50% bio+medical academics generate 56% of papers, while 24% of academics in engineering only produce 10% of the papers (likely their output emerges in other ways).