Introns of Plasmodium falciparum var genes act as transcriptional silencing elements that help control antigenic variations. In transfected episomes, intron silencing of a drug-selectable marker under var promoter control is reversed by the spontaneous deletion of key intron regions. The resulting promoter activation does not affect the transcription of chromosomal var genes.
Laboratory of Malaria and Vector Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892-8132, USA.
