Department of Gene Regulations, Institute of Molecular Biology, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria. svetlana@obzor.bio21.bas.bg|Department of Gene Regulations, Institute of Molecular Biology, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria. svetlana@obzor.bio21.bas.bg|Department of Gene Regulations, Institute of Molecular Biology, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria. svetlana@obzor.bio21.bas.bg