What sort of xenobiochemistry would support information-carrying macromolecules like DNA if only trace amounts of phosphorus were present on an alien planet? Since the phosphoribosyl backbone of DNA would not be possible in such an environment, what might be a viable substitute?
There is a rather controversial example of bacteria here on earth that used Arsenic instead of Phosphorus
Subsequent studies refuted the claim, but it's a starting point. It's plausible that a DNA like molecule could form with Arsenic instead of Phosphorus as Arsenic is directly under Phosphorus on the periodic table. This means it has the same number of valence electons, which is important in molecular bonding, and similar physical properties.
Arsenic is a semi-metal, you could probably use Nitrogen in the same way as both Phosphorous and Nitrogen are non-metals.