The DNase II, Mg2+ procedure was used to fractionate chromatins of a heterotic maize hybrid and parental inbred seedlings. DNase II has been reported to digest DNA at open regions where transcription is occurring, i.e., "euchromatin". Therefore the Mg2+-soluble fraction released by this procedure is thought to be a suitable model for the euchromatin portion of the genome, both in quantity released (a measure of chromatin condensation via nuclease accessibility) and in the quantitative and qualitative analysis of chromatin components. The small Mg2+-insoluble fraction released can be pooled with the considerable digestion-resistant portion of the genome to serve as a model for heterochromatin.
Table 1 shows the results for hybrid and inbred chromatins after 90 min. of digestion. In terms of heterosis, the results are difficult to interpret due to the absence of strong correlations with the quantity of DNA in the hybrid fractions. The amount of Mg2+-soluble "euchromatin" released from hybrid chromatin was intermediate to those of the inbreds, and was similar to that of FRN28, the more vigorous inbred. This does not strongly support a model of heterosis by a mechanism of chromatin conformation, but may do so on the basis of greater transcriptional and regulatory efficiency of the hybrid. This may be supported by data in Table 1, which also shows that the hybrid putative "euchromatin" fraction contains 1) relatively more protein and RNA than DNA than the inbreds; 2) higher proportions of protein and RNA than unfractionated hybrid chromatin; and 3) relatively larger proportions of protein and RNA than the inbred "euchromatin" fractions. Therefore, this procedure isolated maize chromatin fractions that are distinct in composition. Furthermore, the hybrid Mg2+-soluble fraction has unique features which distinguish it from chromatins of the parental inbreds.
Table 1. DNA:Protein:RNA ratios of chromatin fractions from parental maize inbred and F1 heterotic hybrid
S. E. Palmer and V. Ulrich
Return to the MNL 59 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page