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A 2810 bp DNA fragment containing the beta-isopropylmalate dehydrogenase gene of the dimorphic yeast Yarrowia lipolytica has been sequenced. The sequence contains an open reading frame of 405 codons, predicting a protein of 43,366 molecular weight. Protein sequence homology with the polypeptide encoded by the LEU2 gene of Saccharomyces cerevisiae is 64%, whereas DNA sequence homology is 61%. The 5'- and 3'-flanking regions of the Y. lipolytica LEU2 gene share only some general structural features common to genes of S. cerevisiae such as the presence and location of TATA boxes, CAAT boxes, CACACA repeats, the lack of G residues in the 5'-untranslated region and 3'-transcription terminators. Transcription of a 1.4 kb mRNA begins at a small cluster of sites approximately 40 base pairs before the initial ATG.

After conjugation in the ciliated protozoan, Oxytricha, polytene chromosomes are formed during the development of a macronucleus from a micronucleus. Here we report a microscopic study of these chromosomes and an analysis of their DNA. The polytene chromosomes of Oxytricha bear a strong morphological resemblance to the polytene chromosomes of the Dipteran salivary gland. The nucleus of a developing macronuclear anlage contains 120 +/- 2 polytene chromosomes and each chromosome has an average of 81 hands; a total of about 10,000 bands per nucleus. At a later stage in development, the number of bands per chromosome is reduced by a factor of four, presumably due to fusion of adjacent bands. The polytene chromosomes then break up into their constituent bands, each of which is encased in a vesicle. There are about 2,700 vesicles per nucleus.--During the growth of polytene chromosomes, there is a change in the relative proportion of sequences in the DNA. The DNA from polytene nuclei has a buoyant density of 1.695 g/cc, significantly lighter than the density of the original micronuclear DNA (1.698 G/cc to 1.702 g/cc). We interpret this buoyant density change to be the result of differential replication of DNA sequences during polytene chromosome growth. A second change in DNA composition occurs after the polytene stage of development, shown by a shift in buoyant density to 1.701 g/cc in the DNA of the mature macronucleus. During this second process, the molecular weight of the DNA is reduced from greater than 50 x 10(6) daltons to about 2 x 10(6) daltons.