The US’s biggest shareholder advisory firm sued the Securities and Exchange Commission over new guidelines meant to give investors more transparency into how they make their recommendations on director elections and other corporate-governance ballots.

The lawsuit by Rockville, Maryland-based Institutional Shareholder Services is at least the third recent legal action against the SEC by businesses it regulates, a sign that suing a company’s regulator — an uncommon and aggressive tactic — is becoming less taboo as the SEC tries to flex its muscles.