[Cytochemical studies of puffs of isolated salivary-gland-chromosomes of Chironomus]

Features of in vitro puffing and RNA synthesis in polytene chromosomes of Chironomus

In order to elucidate the causal relation between RNA synthesis and puffing in polytene chromosomes, salivary gland nuclei and chromosomes from Chironomus tentans and C. pallidivittatus (Diptera, Chironomidae) were isolated in the native state under controlled and optimized conditions. The behavior of the polytene chromatin in vitro as compared to that in vivo was studied morphologically and autoradiographically. It was shown that the absence of cytologically visible puffing does not necessarily indicate transcriptional quiescence. Under suitable conditions all potential puff sites incorporate the radioactive precursor simultaneously. This can be seen in vivo and even more clearly in chromosomes decondensed by 340-400 mM NaCl in vitro. Potential puff sites therefore appear to be permanently equipped with RNA-polymerases, and there is always a basic low level of transcriptive activity. Experimental shrinking of Balbiani rings and other puffs by dimethylsulfoxide (DMSO) or heat shock in vivo, leaves the genes in the collapsed puffs still transcriptionally active, although to a reduced extent. Light optically visible puffing is therefore not a necessary prerequisite for RNA synthesis, while, conversely, RNA synthesis is required for puff formation.

The control of puffing by ions - the Kroeger hypothesis: a critical review

We have re-examined the several papers which appear to us to represent the principal lines of evidence for what we call the Kroeger hypothesis. To do this we have stated this hypothesis in its simplest, most concrete form, a form that has been repeatedly and forceably enunciated in the literature (Kroeger, 1963a, 1965, 1966, 1967, 1968; Kroeger and Lezzi 1966; Lezzi and Frigg, 1971). The evidence suggests to us that ecdysone's effect on puffing is probably not mediated by the [K+]/[Na+]. While such a model cannot, even now, be excluded, we see little reason to believe in it. We take the general issues raised by Kroeger's ideas very seriously. Nucleoprotein complexes are exquisitely sensitive to changes in salt concentration and ionic selectivity is a well-known property of proteins (and of ion-exchangers in general, see, for example, Diamond and Wright, 1969). Thus it might not be shocking if cells utilized this specificity in some general control over chromsosome structure, perhaps a second-layer of control superimposed upon other transcriptional controls. Therefore it is our feeling that Kroeger's data merits very careful and critical study, the more so because the experiments involved are intrinsically difficult. It is in this vein that we have tried to review Kroeger's data.