Comparative organization of active transcription units in Oncopeltus fasciatus.

We have analyzed electron micrographs of chromatin-associated fiber arrays from embryos of the milkweed bug, Oncopeltus fasciatus. The analysis has revealed that the arrays have highly ordered patterns of fiber spacings and lengths. These patterns support the interpretation that the fibers are nascent RNA with associated proteins (RNP fibers) which have resulted from transcription of the DNA in the underlying chromatin segment. In particular, the patterns indicate that the chromatin underlying each array is delimited by specific sites for initiation and termination of transcription. We apply the term transcription unit to a chromatin segment thus bounded. The analysis has further revealed that transcription units can be grouped into two principal classes--ribosomal and nonribosomal. Active transcription units of these two classes differ in DNA content, in their proximity to other active transcription units, and in their chromatin morphology. For certain developmental stages, fiber frequencies (that is, the nubmers of fibers per mum of chromatin) are also useful in distinguishing ribosomal from nonribosomal arrays. The most definitive of the above classification criteria is chromatin morphology as observed under our preparative conditions. We propose that term rho chromatin for the unbeaded or smooth chromatin that underlies nascent ribosomal RNP fibers. DNA in rho chromatin has a calculated packing ratio of approximately 1.2 mum of B structure DNA per mum of chromatin. Nu chromatin is used to designate the beaded chromatin for which we calculate a DNA packing ratio of 1.6-2.3 in our preparations. This calculation for nu chromatin is based on the inference that the beads are nucleosomes (nu bodies, PS particles, unit particles). The beaded morphology is observed between fibers of nonribosomal transcription unit as well as for most fiber-free chromatin. The detection of specific sites of transcriptional initiation and termination and the classification of transcription units can provide a basis for further analysis of transcriptional control.