Eukaryotic cells can display resistance to a wide range of natural-product chemotheraputic agents by the expression of P-glycoprotein (pgp), a putative plasm membrane transporter that is thought to mediate the effiux of these agents from cells. We have identified, in cells selected for multidrug resistance with actinomycin D, a mutant form of pgp that contains two amino acid substitutions within the putative sixth transmembrane domain. In transfection experiments, this altered pgp confers a cross-resistance phenotype that is altered significantly from that conferred by the normal protein, displaying maximal resistance to actinomycin D. These results strongly implicate the sixth transmembrane domain in the mehanism of pgp drug recognition and efflux. Moreover, they indicate a close functional homology between pgp and the cystic fibrosis transmembrane regulator in which the sixth transmembrane domain has also been shown to influence substrate specificity.Multidrug resistance (mdr) remains a major obstacle in the treatment of neoplasia. Resistance to vinca alkaloids, colchicine, anthracyclines, and actinomycin D can be conferred to eukaryotic cells by the expression of P-glycoprotein (pgp), a plasma membrane transporter that is thought to cause the efflux of these agents by an ATP-dependent mechanism (1). Pgp contains 12 putative transmembrane and 2 ATP binding domains and is a member of a superfamily of membrane transport proteins that have recruited this pump architecture to move vastly diverse substrates across membranes (1, 2). Although pgp substrates are generally lipophillic, they are structurally and functionally dissimilar, and the mechanism by which pgp can mediate the efflux of these different compounds remains unknown (3,4

In multidrug resistance, which is observed clinically and in tissue culture, cells that are challenged with certain cytotoxic drugs develop resistance not only to the selective agent but also to other, seemingly unrelated, agents. The multidrug-resistant phenotype is associated with DNA sequence amplification and with the overproduction of a number of cytosolic and membrane glycoproteins. The differential amplification and altered expression of at least two related genes, termed multidrug-resistant associated genes has been shown in multidrug-resistant Chinese hamster cells. In multidrug-resistant mouse and human cells, genes homologous to those in Chinese hamster cells are also amplified. The level of expression of these genes varied and did not correlate with their copy number. Furthermore, in Chinese hamster cells, the development of resistance to a single drug and multidrug resistance were closely related, but uncoupled, events. The overexpression of the multidrug-resistant genes was better correlated with the degree of resistance to the selective agent than it was with the extent of multidrug resistance.

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We carried out cytogenetic studies of four Chinese hamster, mouse, and human cell lines selected for high levels of resistance (500-to 4,000-fold) to vincristine (VCR) by a multistep selection procedure. All cells examined contained gene amplification-associated metaphase chromosome abnormalities, either homogeneously staining regions (HSRs), abnormally banding regions (ABRs), or double-minute chromosomes (DMs); control actinomycin D-and daunorubicin-resistant hamster lines did not exhibit this type of chromosomal abnormality. VCR-resistant Chinese hamster sublines exhibited both increased synthesis of the protein V19 (Mr 19,000; pl = 5.7) and increased concentrations of V19 polysomal mRNA. When VCR-resistant cells were grown in drug-free medium, level of resistance, synthesis of V19, and amount of V19 mRNA declined in parallel with mean length of the HSR or mean number of DMs per cell. Cross-resistance studies indicate that VCR-resistant cells have increased resistance both to antimitotic agents and to a wide variety of agents unrelated to VCR in chemical structure and/or mechanism of action. Our studies of tubulin synthesis in Chinese hamster cells indicate no overproduction of tubulin or presence of a mutant tubulin species. Comparison with antifolate-resistant Chinese hamster cells known to contain amplified dihydrofolate reductase genes localized to HSRs or ABRs strongly suggests that the HSRs, ABRs, or DMs of the Vinca alkaloid-resistant sublines likewise represent cytological manifestations of specifically amplified genes, possibly encoding V19, involved in development of resistance to VCR.

R-loop and restriction mapping procedures reveal the organization of coding regions at each end of the giant rDNA palindrome of Physarum polycephalum. A 19S coding region of 2.10 +/- 0.21 kb is located at each end of a very long central spacer (35.64 +/- 2.08 kb). An internal spacer of 1.66 +/- 0.12 kb lies distal to the 19S gene. The 5.8S rRNA coding region is located in this spacer. The 26S gene lies distal to the internal spacer. The 26S gene is unusual among those of eukaryotes in that it consists of 3 coding regions (alpha, beta and gamma) interrupted by 2 intervening sequences. The 26S alpha (most central) coding segment of 2.41 +/- 0.33 kb is separated from the 26S beta segment by an intervening sequence of 0.68 +/- 0.13 kb. The 26S beta segment (0.70 +/- 0.11 kb) is separated from the most distal 26S gamma segment (0.59 +/- 0.14 kb) by an intervening sequence of 1.21 +/- 0.14 kb. The 2 intervening sequences are present in at least 88% of ribsomal genes from active plasmodia, indicating that genes containing these sequences are transcribed. The rDNA termini contain a heterogeneous region which varies in length by +/- 300 base pairs.