EPIGENETIC INSTABILITY AND TRANS-SILENCING INTERACTIONS ASSOCIATED WITH AN SPT /AC T-DNA LOCUS IN TOBACCO/

Progeny of tobacco line 2853.6, which carries a streptomycin phosphotr ansferase (SPT) gene interrupted by the maize element Activator (Ac), were selected for streptomycin resistance (Spr) because of germinal Ac excision. Some events gave rise to Spr alleles that were unstable and exhibited a mottled phenotype on streptomycin-containing medium due t o somatic loss of SPT function. This instability was most pronounced i n one particular line, Spr12F. Other Spr alleles rarely exhibited sile ncing of SPT. Streptomycin-sensitive Spr12F plants were recovered, and crosses were performed with other, more stable Spr lines. A high prop ortion of the resulting heterozygous progeny were silenced for SPT exp ression. The silenced state was heritable even after the Spr12F allele segregated away. No correlation could be made between silencing and m ethylation of the SPT gene. Structural analysis of allele Spr12F showe d that the SPT gene from which Ac had excised was flanked by direct re peats of Ar. A search was carried out among 110 additional Spr alleles for new independent unstable alleles, and four were identified. All o f these alleles also carried an SPT gene flanked by direct repeats of Ac. Thus, there is a strong correlation between this structure and ins tability of SPT expression.