Balbiani Ring 

 The polytene chromosomes of insects have extraordinary utility in the investigation of chromosomal structure and function. Polytene chromosomes display the phenomenon of puffing in which the chromatin associated with chromosomal bands is decondensed. In the larval salivary glands of the midge Chironomus, several puffs become very large and are described as Balbiani rings . Balbiani is the cytologist who discovered polytene chromosomes in 1881. The visible aspect of a puff is essentially the complex of RNA and protein that accumulates as a result of vigorous transcriptional activity. Among the gene products encoded by the Balbiani rings are the glue proteins required to attach the midge pupa to its substrate. All of the Balbiani ring genes are expressed coordinately and maximally during the second larval instar. Then they show differential expression in prepupae (1). The most detailed ultrastructural analysis has been performed on Balbiani ring 2 in chromosome IV of Chironomus tentans. Although the polytene chromosome band itself contains more than 470 kbp of DNA, the major gene is 37 kbp long and contains two very well-defined transcription units that are differentially expressed at the prepupal stage. Electron microscopy detects the first engaged RNA polymerase molecule at the approximate start site of transcription and the last RNA polymerase at the site of termination (2, 3). Within each puff, the Balbiani ring gene forms a loop of transcriptionally active chromatin with attached ribonucleoprotein complexes. As synthesis proceeds, the nascent transcripts fold up into compact ribonucleoprotein complexes that head toward the nuclear pores when they are exported to the cytoplasm. Each 37-kb pre-mRNA transcript is processed in the nucleus by staged association with specific proteins (4).

Contour measurements of chromosomal structure in the puff indicate that the chromatin is fully extended into an array of nucleosomes 10 nm in diameter at the site of transcription, whereas once transcription is completed, chromatin coils back up into a 30-nm diameter fiber, which is then finally packaged into a supercoiled loop. Upstream of the start site of transcription is a region free of nucleosomes, presumably corresponding to the promoter, and compacted chromatin fibers are further upstream and downstream of the gene. Immunologic analysis allows defining the structural components of chromatin on active and inactive segments of the Balbiani ring. Surprisingly, proteins, such as histone H1 and the core histones, remain on chromatin even while it is actively transcribed (5). Thus keen observation allows a rather complete ultrastructural picture of the transcription process.

References

1.U. Lendahl and L. Weislander (1987) Develop. Biol. 121, 130–138. 

2.B. Bjorkroth, C. Ericsson, M. M. Lamb, and B. Daneholt (1988) Chromosoma 96, 333–340. 

3.C. Ericsson et al. (1989) Cell 56, 631–639. 

4.H. Mehlin, B. Daneholt, and U. Skoglund (1992) Cell 69, 605–613. 

5. C. Ericsson, U. Grossbach, B. Bjorkroth, and B. Daneholt (1990) Cell 60, 73–83.