Mutator transposon insertions within maize genes often provide a novel outward reading promoter.

This article focuses on the characterization of transcripts arising from Mutator (Mu) transposon insertions within maize genes, particularly investigating the activity of an outward-reading promoter within Mu elements. Analyzing 33 Mu insertion alleles from the UniformMu maize population, the study found that many alleles produce two distinct transcripts: one initiating at the wild-type gene transcription start site (TSS) and another initiating within the Mu element itself via an outward-reading promoter. This promoter activity occurs in multiple Mu element types and orientations and often results in gene expression levels and tissue-specific patterns similar to those of the wild-type gene, suggesting that the Mu promoter acts as a minimal promoter responsive to gene cis-regulatory elements. These findings have implications for maize functional genomics, indicating that Mu insertions may not always create simple loss-of-function alleles but can produce complex transcripts that potentially complement gene function, thereby influencing the interpretation of mutant phenotypes in Mu-tagged populations.