Phosphorus is central to all life on Earth, being a critical component of DNA, cell membranes, and metabolic systems (via ADP/ATP). However, it's not really all that common, especially compared to the classic CHON (carbon, hydrogen, oxygen, nitrogen), which make it a limiting nutrient for the biosphere. So if some alien biosphere could reduce it's dependence on phosphorus, that would be useful!
Some time ago, there was a report of bacteria suspected of using arsenate instead of phosphate in their DNA. As far as I understand the literature, that has been pretty thoroughly debunked, but the question still remains: could other polyatomic ions substitute for phosphorus in different roles?
The article Why Nature Chose Phosphateslays out several reasons that phosphates are useful, and makes the claim that no other option can fill all of the roles that phosphate does. But that doesn't seem to rule out the option of a variety of other ions filling in in different places, to reduce or possibly even eliminate the need for phosphorus after all.
The obvious option to me for several uses is sulfite, which can also form bridges and chains via sulfite ester bonds; the reduced charge on sulfites (-2 rather than -3) means that, e.g., a sulfite-backboned DNA equivalent would no longer by an acid, but I'm not sure how horrendous a problem that would be--it is also true of PNA, after all. Maybe it just needs to work on a non-polar solvent, or maybe there is some other modification that can make it acidic again?
Additionally, sulfolipids, which retain their negatively-charged hydrophillic heads just like phospholipids, so perhaps phosphorus could be replaced in cell membranes as well?
To summarize: it seems that sulfur and nitrogen-based structures could plausibly stand in for phosphorus in a few places; sulfur is less abundant than phosphorus in Earth's crust, but more abundant overall, so maybe that would be useful elsewhere in the universe (like, say, Io).
So, the actual question: Are sulfur and nitrogen indeed plausible stand-ins for phosphorus? If not, are there other options? And just how much phosphorus can actually be replaced? (Or, equivalently, for what functions can phosphorus not be replaced?)