Major Egr3 isoforms are generated via alternate translation start sites and differ in their abilities to activate transcription

In previous studies, we detected a major, unidentified Egr response element (ERE) binding complex in brain extracts. We now report that this complex contains a truncated isoform of Egr3 generated by use of an alternate translation start site at methionine 106. Furthermore, the ERE binding complex previously thought to contain full-length Egr3 includes several isoforms generated by initiation at other internal methionines. Full-length and truncated (missing residues 1 to 105) Egr3 isoforms differ in the ability to stimulate transcription directed by a tandem repeat of two EREs but not by a single ERE. Taken together, our results indicate that alternative translation start sites are used to generate Egr3 isoforms with distinct transcriptional properties.