I would prefer to write the attribute for each member, if you want to save lines of code you could write them inline.

<System.Runtime.Serialization.DataMember> Private mLastUpdated As DateTime
<System.Runtime.Serialization.DataMember> Private mLastUpdatedBy As String
<System.Runtime.Serialization.DataMember> Private mClearanceRequired As Int64
<System.Runtime.Serialization.DataMember> Private mClearanceIsRequired As Boolean

And since you're saving lines of code, why could use auto-implemented properties por LastUpdated, LastUpdatedBy and ClearanceRequired.