Localization of histone gene transcripts in newt lampbrush chromosomes by in situ hybridization

Denatured 3H-labelled DNAs containing histone gene sequences originating from the echinoderms Psammechinus miliaris, Echimus esculentus and Strongylocentrotus purpuratus have been in situ hybridized to RNA transcripts on newt lampbrush chromosomes. Autoradiographs of the hybridized lampbrush preparations show labelling restricted to four or fewer lateral loop pairs all lying within the heteromorphic regions of chromosome I, also one or two loop pairs on chromosome VI, one loop pair on chromosome X and one loop pair on chromosome XI. For oocytes from a single newt, coincident label distribution is found with DNA's of diverse echinoderm origin; however different newts show some specific individual diversity in label distribution, including heterozygosity in the case of loops on bivalents VI and X. The more conspicuously labelled loops, particularly those on chromosome I, show a pattern of labelling which is explicable if the newt histone DNA sequences are confined to short intercepts of lateral loop axis. Transcription is initiated prior to the histone DNA sequences, proceeds through the histone DNA sequences, and beyond, and the histone RNA sequences are cut from the transcripts before the termination of transcription.