Aberrant pre-mRNA maturation is caused by LINE insertions into introns of the white gene of Drosophila melanogaster.

Insertional mutagenesis screens have provided thousands of mutant alleles for analysing genes of varied functions in Drosophila melanogaster. We here document mechanisms of insertional mutagenesis by a LINE element, the I factor, by determining the molecular structure of RNAs produced from two alleles of the white gene of D.melanogaster, wIR1 and wIR6. These alleles result from insertion of the I factor into introns of the gene. We show that sequences present within the element direct aberrant splicing and termination events. When the I factor is inserted within the white first intron it may lead to the use of a cryptic 3' splice site which does not contain the dinucleotide AG. This splicing gives rise to a chimeric messenger RNA whose synthesis is controlled differently in tissues where the mutated gene is expressed. When the I factor is inserted within the white last intron it induces synthesis of truncated mRNAs. These results provide, for the first time, mechanisms for I factor insertional mutagenesis. They are discussed in the more general context of RNA processing in Drosophila and the evolution of eukaryotic gene introns.