Tam3 produces a suppressible allele of the DAG locus of Antirrhinum majus similar to Mu-suppressible alleles of maize

Tam3 from Antirrhinum majus belongs to the Ac/Ds family of transposable elements. An allele of the DAG locus of Antirrhinum (dag::Tam3), which is required for chroroplast development and leaf palisade differentiation, has been generated by Tam3 insertion into the untranslated leader sequence of the gene. This allele gives rise to a cold-sensitive phenotype, where mutant tissue containing wildtype revertant somatic sectors is observed in the leaves of plants grown at 15 degrees C, while leaves of plants grown at 25 degrees C appear near wild-type. The temperature sensitivity of dag::Tam3 results from expression of the DAG locus responding to the activity of the transposable element, the transposition of which is very sensitive to growing temperature. Genetic suppression of Tam3 transposition, using the STABILISER locus, also results in suppression of the dag mutant phenotype. dag::Tam3 represents a Tam3-suppressible allele similar to those described for Mu transposons in maize. Suppression of the dag mutant phenotype in response to element inactivation appears to result from use of an alternative promoter at the 3' end of the Tam3 element. The production of suppressible alleles by an Ac-like element is discussed in relation to the mutagenic potential of plant transposons in producing complex genetic diversity.