CRITICAL ELECTROLYTE CONCENTRATION OF CHROMATIN IN POLYTENE CHROMOSOMES OF TRICHOSIA PUBESCANS (DIPTERA, SCIARIDAE)

The binding of toluidine blue molecules to nucleic acid phosphates und er conditions of competition with Mg2+ ions was studied in Trichosia p ubescens polytene chromosome regions differing in composition and orga nization (RNA-rich areas and puffs, DNA puffs, heterochromatin). The a im was to find variability in the Mg2+ concentration at which metachro masy was completely prevented (= critical electrolyte concentration; C EC) and that could reflect differences at the level of nucleoprotein c omplexes in particular chromosome regions and developmental phases. Al though high CEC values have been found in tightly packed chromatin suc h as that of heterochromatin zones, the CEC values for the other regio ns have proven to be affected not only by availability and proximity o f DNA phosphates but also by RNA richness and other factors. Among the se, changes in DNA geometry and packing state have been assumed for ex plaining increased values after RNA removal in DNA puffs and RNA-rich zones. Based on CEC values it has been suggested that alterations at t he level of nucleoprotein complexes may occur in puffs before they are morphologically detectable. CEC results for polytene chromosomes were thus revealed to vary considerably with slight variations in nucleopr otein composition and organization. However, since a complex and appar ently contradictory rationale has to be used for explaining part of th e results, CEC is not recommended as a useful tool for extensive and c omparative studies of this particular model.