Putnam Mortgage Opportunities Co., which invests in residential mortgage-backed securities guaranteed by government agencies, has filed an initial public offering form with the Securities and Exchange Commission to raise $300 million in capital. The real estate investment trust, which is managed externally by Boston-based Putnam Advisory Co., had about $121 billion in assets under its umbrella as of Dec. 31, according to SEC filings. While Putnam may pursue non-government RMBS, commercial mortgage-backed securities and asset-backed securities, it’s main focus is investing in agency RMBS guaranteed by Fannie Mae and Freddie Mac. In its filing, Putnam Mortgage did not indicate how many shares will be offered or for what price. The firm is applying to list its stock under the ticker symbol PMOC. Barclays Capital (BCS) will serve as the underwriter of the transaction. Write to Kerri Panchuk.