We have recently shown that magnification, an increase in the number of ribosomal RNA genes (rDNA) in gametes produced by rDNA-deficient flies, can occur in female Drosophila if they have a Y ...chromosome. We now have tested several X-Y translocation and recombinant chromosomes to determine which parts of the Y chromosome are necessary for magnification to occur in females. Our data indicate that the required region is the distal part of the long arm of the Y chromosome, YL. We have also used X-Y translocation chromosomes to study magnification of rDNA-deficient X chromosomes in males. Our data show that the region of the Y chromosome from the distal end of the nucleolus organizer through the centromere is not required for magnification in males. The frequency of magnification in males with rDNA-deficient Y fragments is comparable to that produced by Ybb-, a chromosome that has often been used to produce magnification in males. These results demonstrate that the Ybb-chromosome is not uniquely effective in causing magnification to occur in males. The results of these studies imply that sequences present on YLare required for magnification to occur in females; these sequences are probably also required for magnification in males. Since unequal sister chromatid exchange has been implicated as the major mechanism of ribosomal gene increase during magnification, the YLsequences required for magnification may be involved in encoding or regulating products needed for sister chromatid recombination in germ-line cells.
The genetically induced increase in the number of 18S + 28S ribosomal genes known as magnification has been reported to occur in male Drosophila but has not previously been observed in females. We ...now report that bobbed magnified (bbm) is recovered in progeny of female Drosophila carrying three different X bobbed (Xbb) chromosomes and the helper XYbb chromosome, which is a derivative of the Ybb- chromosome. Using different combinations of bb or bb+ X and Y chromosomes, we show that magnification in females requires both a deficiency in ribosomal genes and the presence of a Y chromosome: X/X females that are rDNA-deficient but do not carry a Y chromosome do not produce bbm; similarly, X/X/Y females that carry a Y chromosome but are not rDNA-deficient do not produce bbm. Bobbed magnified is only recovered from rDNA-deficient X/XY, X/X/Y or XX/Y females. We have also found that females carrying a ring Xbb chromosome together with the XYbb- chromosome do not produce bbm, indicating that ring X chromosomes are inhibited to magnify in females as in males. We postulate that the requirement for a Y chromosome is due to sequences on the Y chromosome that regulate or encode factor(s) required for magnification, or alternatively, affect pairing of the ribosomal genes.--These studies demonstrate that magnification is not limited to males but also occurs in females. Magnification in females is induced by rDNA-deficient conditions and the presence of a Y chromosome, and probably occurs by a mechanism similar to that in males.
Previous studies indicate that genes from only one of the cell's nucleolus organizers undergo multiple rounds of DNA replication in polytene cells of Drosophila. This report presents evidence that ...this effect is mediated by a function that is associated with the ribosomal genes of the dominant or replicating X chromosome. This function can act in trans to result in replication of the ribosomal genes on the recessive X chromosome in flies that are bobbed for the dominant X chromosome. In these cases, ribosomal genes from both chromosomes undergo polytenization. Heterochromatic regions that flank the nucleolus organizer have little or no effect on nucleolar dominance. In addition, deletion of the compensatory response (cr+) locus does not affect the dominance, suggesting that ribosomal gene compensation and nucleolar dominance in polytene cells of Drosophila are separate genetic phenomena.
Tartof showed that ribosomal gene magnification in Drosophila was inhibited in a ring X chromosome. The present studies extend this observation by showing that ring X chromosomes are lost meiotically ...in male Drosophila undergoing ribosomal gene magnification as evidenced by the recovery of a lower number of ring-bearing progeny under magnifying conditions compared with nonmagnifying conditions. Associated with ring chromosome loss is a highly significant increase in the number of double-sized dicentric ring chromosomes in meiotic cells from magnifying males. These observations explain the failure of ring X chromosomes to magnify and imply that magnification in rod chromosomes occurs via a mechanism of unequal sister chromatid exchange. Our results support the hypothesis that the primary event of magnification is a sister chromatid exchange in the rDNA, that the frequency of sister strand exchanges is increased in magnifying flies, that a significant number of exchanges in magnifying flies occurs meiotically and that some of the exchanges are nonreciprocal. We have also found that autosomal mutations can affect both the frequency of abnormal ring structures and the ability of ring X chromosomes to magnify.
The genes coding for the 18S and 28S rRNAs in D. melanogaster were examined using Southern transfers of DNA from diploid or polytene tissue. A ribosomal gene repeat 12 kb in length is present in DNA ...from diploid tissue of males and is the major repeat on the Y chromosome. This repeat is present in low amounts on the X chromosome, which contains major repeats of 17 and 11.5 kb. In polytene nuclei of males, the 12 kb band is disproportionately replicated, and only a very low amount of the 11.5 kb repeat and no 17 kb repeat are detected. Polytene nuclei of females contain reduced amounts of the 17 kb repeat relative to the 11.5 kb repeat. This disproportionate replication of specific ribosomal gene repeats suggests that polytenization of the rDNA may involve an extrachromosomal mechanism. Evidence that genes from only one nucleolus organizer are replicated during polytenization in X/Y and X/X flies is discussed. A method for analyzing DNA from tissue of individual larvae was developed to test for population heterogeneity in ribosomal gene structure. Heterogeneity was observed in the ribosomal genes of three Ore R lines, four other D. melanogaster strains and between males and females of the same strain.
Recent studies indicate that genes from only one nucleolus organizer undergo replication during polytenization of salivary gland cells of X/Y flies. This report presents evidence that this is also ...true X/X polytene cells. Interstrain hybrids were constructed which contain X chromosomes with different rDNA hybridization patterns following digestion with Eco RI, and DNA from diploid or polytene tissue was analyzed using Southern transfers. DNA from diploid tissue of X/X hybrids shows rDNA patterns from both X chromosomes, while DNA from polytene tissue of hybrids shows a pattern characteristic of only one of the X chromosomes. The polytene pattern of the hybrid is independent of whether the dominant X chromosome was derived from the female or male parent. These results suggest that genes from only one nucleolus organizer undergo multiple rounds of replication in X/X polytene cells, and account for the finding that X/O and X/X polytene cells contain similar levels of rDNA without postulating compensation as a ribosomal gene rectification mechanism.
The kinesin family of motor proteins, which contain a conserved motor domain of approximately 350 amino acids, generate movement against microtubules. Over 90 members of this family have been ...identified, including motors that move toward the minus or plus end of microtubules. The Kar3 protein from Saccharomyces cerevisiae is a minus end-directed kinesin family member that is involved in both nuclear fusion, or karyogamy, and mitosis. The Kar3 protein is 729 residues in length with the motor domain located in the C-terminal 347 residues. Recently, the three-dimensional structures of two kinesin family members have been reported. These structures include the motor domains of the plus end-directed kinesin heavy chain Kull, F. J., et al. (1996) Nature 380, 550-555 and the minus end-directed Ncd Sablin, E. P., et al. (1996) Nature 380, 555-559. We now report the structure of the Kar3 protein complexed with Mg.ADP obtained from crystallographic data to 2.3 A. The structure is similar to those of the earlier kinesin family members, but shows differences as well, most notably in the length of helix alpha 4, a helix which is believed to be involved in conformational changes during the hydrolysis cycle.
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