1. Laboratory of Genetic Engineering, Institute of Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 38 Bratislava, Slovak Republic, Institute of Virology, Slovak Academy of Sciences, Dúbravská cesta 9, 845 45 Bratislava, Slovak Republic, and Institute of Biochemistry, Freie Universität Berlin, Thielallele 63, D-14195 Berlin, Germany