"DBInstance" : {
  "Type": "AWS::RDS::DBInstance",
  "Properties": {
    "DBName" : { "Ref" : "DBName" },
    ...
  }
},

"DBInstance" : {
  "Type": "AWS::RDS::DBInstance",
  "Properties": {
    ...
    "DBSecurityGroups" : [{ "Ref" : "DBSecurityGroup" }],
    ...
  }
},