Protein tightly bound near the termini of the Physarum extrachromosomal rDNA palindrome.

Abstract

The genes coding for rRNA in plasmodia of P. polycephalum are arranged palindromically on extrachromosomal rDNA molecules of 61 kb (kilobase pairs). Incubation of mildly extracted rDNA with the 125I Bolton-Hunter reagent results in incorporation of label not removed by SDS, CsCl or various organic solvents. Labeled protein is preferentially associated with terminal rDNA restriction fragments, as detected after gel electrophoresis of the DNA. Antibody reaction with dinitrophenylated protein-rDNA complexes allows visualization of protein located 1-2 kb from the termini, in a region containing multiple inverted repeat sequences and single-strand gaps. DNase I treatment of either rDNA or rDNA termini releases primarily 2 labeled protein bands of 5000 and 13,000 daltons as well as less prominent bands of higher MW. Mechanisms were discussed for involvement of terminal protein in replication of 3'' ends and chromosomal integration of the rDNA.