MAIZE SPM TRANSPOSABLE ELEMENT HAS AN ENHANCER-INSENSITIVE PROMOTER

We have used a transient assay system to investigate the promoter regi on of the maize Suppressor-mutator (Spm) transposable element. All of the sequence required for constitutive promoter activity is confined t o the 0.2-kb sequence upstream from the transcription start site of th e element at nt 209 and designated the upstream control region. The el ement's promoter is weak, lacks a conventional TATA box, and depends o n the presence of multiple, short repetitive sequence elements. The Sp m promoter is quite insensitive to the enhancer sequence of the caulif lower mosaic virus 35S promoter. Enhancer sensitivity can be restored by providing a -30 TATA sequence and removing the G+C-rich sequence en coding the untranslated leader of the element, designated the downstre am control region. Although the downstream control region is without e ffect on Spm promoter activity, it completely inhibits the 35S core pr omoter and markedly inhibits activity of the complete 35S promoter. Th e properties of the Spm element's promoter buffer it from both mutatio nal and position-dependent changes in activity. We suggest that the in herent characteristics of the promoter are part of the genetic mechani sm that controls the element's transposition frequency, ensuring it re mains low and insertion-site independent.