Nucleosome Arrangement on TRNA Genes of Xenopus Laevis

We have examined the chromatin structure of a repeated cluster of tRNA genes of Xenopus laevis, with particular attention to possible phase relationships between nucleosomes and the underlying tDNA sequence. In transcriptionally active liver and cultured kidney cells, essentially all tDNA is packaged into regularly spaced nucleosomes, but no detectable phase relationship exists between nucleosomes and DNA sequence. Only in transcriptionally inactive erythrocytes do nucleosomes on all tDNA repeats occur in one predominant phase. In this phasing arrangement, nucleosomes are regularly spaced with the exception of a 250 base pair gap occurring once per 3.18 kilobase repeat unit. This arrangement suggests that nucleosome phasing may aid in the organization of a specific higher order packaging, with each unit of packaging containing one 3.18 kilobase repeat.