Compile error Changing target of vspackage from 4.5 to 4 in vs2012

Hi,

I have a vspackage that works fine in vs2012  targeting .net 4.5, when I try to change the target to 4 or 3.5 i get compile error,

Please let me know if it is possible to target .net 3.5 or 4 for vspackage  , how i can resolve the compile error when reaching the following line in source code:

  [PackageRegistration(UseManagedResourcesOnly = true)]

Error7 The type or namespace name 'PackageRegistrationAttribute' could not be found (are you missing a using directive or an assembly reference?)VSPackage1Package.cs246VsPackageSetup

Thanks in advance