Phosphorylation of some chromosomal nonhistone proteins in active genes is blocked by the transcription inhibitor 5,6-dichloro-1-beta-D- ribofuranosylbenzimidazole (DRB)

The distribution of rapidly phosphorylated chromosomal proteins between chromosome I, chromosome II + III, chromosome IV, and nuclear sap including the matrix was investigated in salivary gland cells of Chironomus tentans. Chromosome IV, which carries most active nonribosomal genes in the cell, was found to be enriched in four rapidly phosphorylated nonhistone polypeptides (Mr = 25,000, 30,000, 33,000, and 42,000) in parallel with the transcriptional activity rather than with the DNA content of the chromosome. Also the histones H2A and H4 are rapidly phosphorylated but the phosphorylation is proportional to the DNA content of each chromosome sample. The 32P-labeled Mr = 42,000 polypeptide immunologically cross-reacted with an antibody elicited against the transcription stimulatory factor S-II isolated from Ehrlich ascites tumor cells (Sekimizu, K., D. Mizuno, and S. Natori, 1979, Exp. Cell Res., 124:63-72). In addition, indirect immunofluorescence studies on chromosome IV with antisera against the stimulatory factor II revealed a selective staining of the active gene loci. The incorporation of 32P into three chromosome IV nonhistone polypeptides, especially into the Mr = 42,000 polypeptide, was lowered by 70-85% shortly after administration of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), a likely inhibitor of heterogeneous nuclear RNA transcription at initiation level. The possibility of a causal relationship between inhibited phosphorylation of chromosomal proteins and blocked transcription of heterogeneous nuclear RNA genes by DRB is discussed.