A dramatic difference is observed in the intracellular distribution of the high mobility group (HMG) proteins when chicken embryo fibroblasts are fractionated into nucleus and cytoplasm by either mass enucleation of cytochalasin-B-treated cells or by differential centrifugation of mechanically disrupted cells. Nuclei (karyoplasts) obtained by cytochalasin B treatment of cells contain >90% of the HMG 1, while enucleated cytoplasts contain the remainder. A similar distribution between karyoplasts and cytoplasts is observed for the H1 histones and the nucleosomal core histones as anticipated . The presence of these proteins, in low amounts, in the cytoplast preparation can be accounted for by the small percentage of unenucleated cells present. In contrast, the nuclei isolated from mechanically disrupted cells contain only 30-40% of the total HMGs 1 and 2, the remainder being recovered in the cytosol fraction . No histone is observed in the cytosol fraction . Unlike the higher molecular weight HMGs, most of the HMGs 14 and 17 sediment with the nuclei after cell lysis by mechanical disruption . The distribution of HMGs is unaffected by incubating cells with cytochalasin B and mechanically fractionating rather than enucleating them . Therefore, the dramatic difference in HMG 1 distribution observed using the two fractionation techniques cannot be explained by a cytochalasin-B-induced redistribution . On reextraction and sedimentation of isolated nuclei obtained by mechanical cell disruption, only 8% of the HMG 1 is released to the supernate. Thus, the majority of the HMG 1 originally isolated with these nuclei, representing 35% of the total HMG 1, is stably bound, as is all the HMGs 14 and 17 . The remaining 65% of the HMGs 1 and 2 is unstably bound and leaks to the cytosol fraction under the conditions of mechanical disruption . It is suggested that the unstably bound HMGs form a protein pool capable of equilibrating between cytoplasm and stably bound HMGs .Evidence has accumulated that the high mobility group (HMG) chromosomal proteins are required for transcriptionally active chromatin structure . Incubation of chromatin with DNase I preferentially digests transcriptionally active sequences (4,16,34), while selectively releasing the HMGs (15,17,18,33,35) . The preferential DNase I sensitivity of the globin genes in erythrocyte mononucleosomes is abolished by the selective extraction of the nonhistone fraction containing the HMGs, while readdition of purified HMGs restores it (35,36) . The HMGs may also be involved, at least indirectly, in the regulation of differential gene activity during cytodifferentiation since they are themselves developmentally regulated proteins. Cordon and colleagues (7) have shown that the proportions of HMGs 1, 2A, and 2B vary between different chicken tissues and that quantitative changes in HMG proportions are ob-

SUMMARYChanges in multiple forms of acid phosphatase from potato tubers during infection by Phytophthora erythroseptica were detected by polyacrylamide gel electrophoresis. In recently lifted tubers these changes included the appearance of a new molecular form of low electrophoretic mobility. Seasonal studies established that a similar molecular form developed in healthy tubers during storage. An antiserum to acid phosphatase from potato tubers was shown to be specific for the host enzyme. Utilizing a sectional immunofocusing procedure the novel molecular form from infected tissues cross-reacted with the antiserum, suggesting that this component originated from the host.

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