Experimental evidence that the maize Activator (Ac) transposase can act on a non-transposable element promoter to repress reporter gene expression in transient plant assays

The transposase (TPase) encoded by the maize Activator (Ac) transposable element (TE) has been shown to repress the Ac TPase gene promoter (a form of element 'self-repression'). However, there is a lack of current experimental evidence to suggest that the Ac TPase can repress or activate the expression of non-TE gene promoters. Despite certain attributes of the maize Ac element and its TPase that might argue against such a broader, 'second function', that is, in regulating non-TE genes, the hypothesis that Ac TPase regulates gene expression was nevertheless tested in an in vivo tobacco BY-2 cell system using three different (non-TE) CaMV 35S promoters each linked to a gusA reporter gene. Four experiments were conducted; in each, BY-2 cells were co-transfected with two types of constructs: a CaMV 35S-gusA promoter construct (reporter) and an Ac expression construct (effector). In transient assays, a significant reduction in the magnitude of GUS enzyme activity was observed in the presence of an Ac effector (TPase-encoding) construct, below that observed for the 'no effector construct' treatment. A similar effector versus no effector comparison was made by Fridlender et al. [19], who studied Ac element 'self-repression'. This Ac-associated GUS enzyme activity reduction was observed herein specifically for the gusA promoter construct with the 700 bp native CaMV 35S promoter region (a non-TE promoter). This construct contains 34 putative Ac TPase binding site motifs, 71% of which overlap one another on either DNA strand, motifs which also fall into the class of 'CpG islands' in plants. It is suggested that the Ac TPase can bind to such Ac DNA motifs and CpG islands existing within non-TE promoter regions, and can function as a regulator of transcription. Such a transcriptional mode of regulation has not been shown explicitly herein. The implications of the findings are discussed and a genomic (whole genome) regulation model is presented.