Concanavalin A binds to puffs in polytene chromosomes

Abstract

CHANGES in transcriptional activity at defined loci are often correlated with significant local structural changes in the genome¹, and in polytene chromosomes, such changes are thought to be associated with compositional or conformational changes in the protein complement at these particular bands^2,3. Thus, various studies on Balfoiani rings and specific ‘puffs’ in such chromosomes are useful for elucidating the role of defined chromosomal components in both chromosome structure and gene activity. Such studies require specific probes which will allow in situ localisation of a chromosomal component during the various stages of puffing. Antibodies specific to purified histone fractions^4–7, HMG proteins⁸, RNA polymerase⁹ and non-histone protein subfractions¹⁰ have been used in studies on chromatin and chromosome structure. We reported previously that concanavalin A (Con A) specifically binds to three types of non-histone proteins present in chromatin purified from rat liver nuclei and suggested that derivatives of Con A might serve as specific probes to study the in situ organisation of these non-histone proteins¹¹. We have now reacted fluorescein-labelled Con A with polytene chromosomes isolated from different developmental stages of Chironomus thummi and visualised the location of the bound Con A by fluorescence microscopy. We observed that the fluorescent lectin, which has an affinity for glucose- and mannose-containing molecules, specifically bound to the transcriptionally active regions of chromosome IV. The extent of binding of Con A to the Balbiani rings present in regions b and c of chromosome IV is proportional to the size of the respective ring. Our results indicate that glucose- or mannose-containing molecules are present in these Balbiani rings and that the availability of these sugars to interact with Con A can be correlated with the developmental stage of a puff. We suggest that lectins can be useful cytological tools with which to study the in situ organisation of defined chromosomal components during various functional states of the genome.