X0101 EFFECT 33 LIVE 2018-04-19 0000928816-18-000570 485APOS 0000732337 PUTNAM MORTGAGE SECURITIES FUND 002-87634 S000006570 PUTNAM U S GOVERNMENT INCOME TRUST C000017922 Class A Shares S000006570 PUTNAM U S GOVERNMENT INCOME TRUST C000017923 Class B Shares S000006570 PUTNAM U S GOVERNMENT INCOME TRUST C000017924 Class C Shares S000006570 PUTNAM U S GOVERNMENT INCOME TRUST C000017925 Class M Shares S000006570 PUTNAM U S GOVERNMENT INCOME TRUST C000017926 Class R Shares S000006570 PUTNAM U S GOVERNMENT INCOME TRUST C000017927 Class Y Shares S000006570 PUTNAM U S GOVERNMENT INCOME TRUST C000185164 Class T Shares S000006570 PUTNAM U S GOVERNMENT INCOME TRUST C000200295 Class R6 Shares