Biochemical studies of U7 snRNPs and of histone RNA 3' processing

Human replication-dependent histone H3 genes are activated by a tandemly arranged pair of two CCAAT boxes

We have analysed the transcriptional regulation of the human histone H3 genes using promoter deletion series, scanning mutagenesis, specific mutagenesis and electrophoretic mobility-shift assay experiments. The promoters of five of the six examined histone H3 genes showed near-maximal activity at lengths of 133-227 bp: H3/d 198 bp, H3/h 147 bp, H3/k 133 bp, H3/m 227 bp, H3/n 140 bp (exception H3/i). To search for functional cis-elements within these regions, we performed scanning mutagenesis of the two histone H3 promoters H3/k and H3/m. Mutagenesis revealed that the functional framework of the histone H3 promoters consists of a TATA box and two tandemly arranged CCAAT boxes in relatively fixed positions. Alterations of the distance between the CCAAT boxes and of the distance between the CCAAT boxes and the TATA box resulted in significant loss of activity. In electrophoretic mobility-shift assay experiments, the factor CBF (CCAAT-binding factor)/NF-Y (nuclear factor-Y) bound to isolated CCAAT boxes of the H3/k promoter. This suggests that an initiation complex is formed on the histone H3 promoter that has a defined structure and limited flexibility, consisting of two molecules of CBF/NF-Y and further (general or specific) transcription factors.

Aberrant expression pattern of replication-dependent histone h3 subtype genes in human tumor cell lines

We have determined the expression pattern of all 11 human replication-dependent histone H3 genes in three fetal human tissues (bladder, liver, and lung) and in eight human cell lines by RNase protection assay. In the fetal human tissues, all 11 genes were expressed to a varied extent. However, the relative contribution of each gene to the total replication-dependent histone H3 mRNA was rather similar in every tissue type. The expression pattern in the fibroblast cell line IMR 90 was similar to the expression pattern in the three fetal tissues. In contrast, the expression patterns varied substantially in seven tumor cell lines: some genes were not expressed at all, and others were expressed much less than in the fetal tissues or the fibroblast cell line. This aberrant expression was different in each of the cell lines tested. In a transient reporter gene assay using the promoters of 6 of the 11 genes, however, the relative activities of the promoters were similar in all cell lines. This indicates that the aberrant expression pattern in the different tumor cell lines is not due to a differential availability of transcription factors. We conclude that the varied expression pattern of the replication-dependent histone H3 genes in the examined human tumor cell lines is most probably due to epigenetic factors, such as the chromosomal context in the different cell lines.