HIGH-MOBILITY GROUP PROTEINS CHMG1A, CHMG1B, AND CHMGI ARE DISTINCTLYDISTRIBUTED IN CHROMOSOMES AND DIFFERENTIALLY EXPRESSED DURING ECDYSONE DEPENDENT CELL-DIFFERENTIATION

The mammalian high mobility group proteins HMGI/Y and HMG1/2 are thoug ht to play an architectural role in assembly of nucleoprotein structur es. Counterparts to these proteins have recently been found in the cel ls of the Dipteran insect Chironomus. In this report we investigate th e distribution of three abundant HMG proteins in interphase giant chro mosomes of the midge, Chironomus. By means of the indirect immunofluor escence technique the cHMG1b and cHMGI proteins were localized in chro mosomal puffs, suggesting their involvement in the organization of tra nscriptionally active chromatin. In contrast, the highly abundant prot ein cHMG1a was rather uniformly distributed in the chromosomes. The cH MGI protein, but not cHMG1a or cHMG1b, was detected in nucleoli, which may indicate a role in the transcription of ribosomal genes. The regi ons of the interphase chromosomes containing AT-rich DNA did not conta in higher levels of the cHMGI and cHMG1b proteins. A correlation betwe en the specific location of these proteins in chromatin and their synt hesis and turnover rates was observed.