INTRASPECIFIC VARIABILITY OF THE TANDEM REPEATS IN NICOTIANA PUTRESCINE N-METHYLTRANSFERASES

The putrescine N-methyltransferase (PMT) cDNA clone previously isolate d from tobacco encodes a spermidine synthase-like protein with an 11 a mino acid element repeated four times in tandem at the amino terminus. Genomic Southern blot analyses indicated that this N-terminal repeat array is found in tobacco PMTs but absent in Hyoscyamus and Atropa PMT s. A truncated tobacco PMT in which this repeat array was entirely rem oved still retained full enzymatic activity when expressed in Escheric hia coli. Three PMT genes (NsPMT1, NsPMT2, NsPMT3) isolated from Nicot iana sylvestris encode two, five, and nine tandem repeats, respectivel y, in the first exon, but otherwise encode highly conserved proteins. Analysis of PCR fragments amplified from the genomes of N. tabacum and its two probable progenitors shows that one of the nine repeat elemen ts in NsPMT3 was precisely deleted in the corresponding N. tabacum gen e. These results indicate that direct tandem repeats of a 33 bp sequen ce that encodes 11 amino acids of no obvious function were added to th e ancestral Nicotiana PMT gene, and that the tandem repetition was gen etically very unstable, contracting or expanding during evolution of t he Nicotiana species.