Protein-DNA cross-linking reveals dramatic variation in RNA polymerase II density on different histone repeats of Drosophila melanogaster.

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Abstract
In Drosophila melanogaster the five histone genes are within a 5-kilobase region which is repeated 100 times at a single chromosomal site. These 5-kilobase repeats are of two distinct classes, short and long, that differ by approximately 200 base pairs of DNA in the spacer region between the H1 and H3 genes. Since the mRNA-homologous regions of the repeats are highly conserved, one cannot examine differential expression of the repeats by classical hybridization methods. In this study, we assessed their transcriptional activity by measuring in vivo the relative amounts of RNA polymerase II that were cross-linked by UV irradiation to the two different histone repeats. The RNA polymerase II density on the long repeat in Schneider line 2 cells was strikingly lower (10-fold) than the density on the short repeat. The magnitude of this difference cannot be accounted for by reduced transcription of only one or two genes of the repeat. The density of topoisomerase I, an indicator of transcriptional activity, was also much higher on the short repeat than on the long repeat of line 2 cells. In contrast, the RNA polymerase II density was slightly higher on the long repeat than on the short repeat in a second cell line, KcH. The major difference between active (KcH) and inactive (S2) long repeats resides in the H1-H3 nontranscribed spacer. This portion of the spacer may contain a component necessary for expression that can act over a moderate distance and affect multiple genes of the repeat.