Alternating polarity in rRNA genes.

Spread nucleolar material from A. exigua exhibits an unusual arrangement of rDNA cistrons. The cistrons form head-to-head “diamond-shaped” doublets with a short spacer separating the members of the doublet. Single cistrons are usually terminal and arranged tail-to-tail with the adjacent doublet, suggesting the possibility of a repeating unit with a tail-to-tail, “bow-tie” doublet configuration in which the cistrons (average length: 1.44 μm) could be explained by an extraordinary shrinking of the DNA strand during transcription. This suggestion is supported by the estimation of the molecular mass of an early, or the primary, transcription product (2.2 × 106).