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Eight terminally deleted Drosophila melanogaster chromosomes have now been found to be "healed." In each case, the healed chromosome end had acquired sequence from the HeT DNA family, a complex family of repeated sequences found only in telomeric and pericentric heterochromatin. The sequences were apparently added by transposition events involving no sequence homology. We now report that the sequences transposed in healing these chromosomes identify a novel transposable element, HeT-A, which makes up a subset of the HeT DNA family. Addition of HeT-A elements to broken chromosome ends appears to be polar. The proximal junction between each element and the broken chromosome end is an oligo(A) tract beginning 54 nucleotides downstream from a conserved AATAAA sequence on the strand running 5' to 3' from the chromosome end.The distal (telomeric) ends of HeT-A elements are variably truncated; however, we have not yet been able to determine the extreme distal sequence of a complete element. Our analysis covers approximately 2,600 nucleotides of the HeT-A element, beginning with the oligo(A) tract at one end. Sequence homology is strong (>75% between all elements studied). Sequence may be conserved for DNA structure rather than for protein coding; even the most recently transposed HeT-A elements lack significant open reading frames in the region studied. Instead, the elements exhibit conserved short-range sequence repeats and periodic long-range variation in base composition. These conserved features suggest that HeT-A elements, although transposable elements, may have a structural role in telomere organization or maintenance.The ends of eukaryotic chromosomes analyzed to date contain multiple repeats of very short, simple G-rich sequences (e.g., TTAGGG). These repeats are now considered the telomere sequences. There is evidence that the G-rich repeats can be added by telomerase, an enzyme that uses an RNA template to add copies of the telomere repeat to the chromosome end (5). Just internal to the telomere repeats, eukaryotic chromosomes have more complex sets of repeats (3,4,32) which are called telomere-associated repeats; however, their constant association with chromosome ends raises the possibility that telomere-associated sequences may be responsible for some of the functions that cytologists have suggested for the telomere. These possible functions include mediation of telomere-telomere and telomere-nuclear lamina interactions (14, 24). Thus, there is a possibility that telomere-associated sequences play a role in chromosome pairing at meiosis and/or in maintenance of the threedimensional organization of chromosomes within the nucleus.No simple telomerase-generated sequences have been found on Drosophila chromosomes. The failure to find simple repeats is not proof that they do not exist; however, it is possible that Drosophila species have lost this part of the ancestral telomere. On the other hand, Drosophila chromosomes do have larger, more complex telomere-associated sequences which may be evolutionarily rel...

Abstract. Twelve monoclonal antibodies have been raised against proteins in preparations of Z-disks isolated from Drosophila melanogaster flight muscle. The monoclonal antibodies that recognized Z-hand components were identified by immunofluorescence microscopy of flight muscle myofibrils. These antibodies have identified three Z-disk antigens on immunoblots of myofibrillar proteins. Monoclonal antibodies c~:1-4 recognize a 90-100-kD protein which we identify as o~-actinin on the basis of cross-reactivity with antibodies raised against honeybee and vertebrate ot-actinins. Monoclonal antibodies P:l-4 bind to the high molecular mass protein, projectin, a component of connecting filaments that link the ends of thick filaments to the Z-band in insect asynchronous flight muscles. The anti-projectin antibodies also stain synchronous muscle, but, surprisingly, the epitopes here are within the A-bands, not between the A-and Z-bands, as in flight muscle. Monoclonal antibodies Z(210):1-4 recognize a 210-kD protein that has not been previously shown to be a Z-band structural component. A fourth antigen, resolved as a doublet (,o400/600 kD) on immunoblots of Drosophila fibrillar proteins, is detected by a cross reacting antibody, Z(400):2, raised against a protein in isolated honeybee Z-disks. On Lowicryl sections of asynchronous flight muscle, indirect immunogold staining has localized ot-actinin and the 210-kD protein throughout the matrix of the Z-band, projectin between the Z: and A-bands, and the 400/600-kD components at the I-band/Z-band junction. Drosophila ol-actinin, projectin, and the 400/600-kD components share some antigenic determinants with corresponding honeybee proteins, but no honeybee protein interacts with any of the Z(210) antibodies.T HE Z-band is an electron-dense structural component of striated muscle. It serves as an attachment site for thin filaments and transmits tension between neighboring sarcomeres during contraction. Electron micrographs of both vertebrate muscle and insect fibrillar muscle show Z-bands with a highly ordered, almost crystalline, appearance in cross section (for reviews see 1, 4, 12, 40, 42). Several Z-band proteins have been identified from both vertebrate and insect species (2, 3, 6, 7, 18, 20-22, 25, 29, 30, 33-38); however, the manner in which these proteins are organized within the Z-band lattice is poorly understood. Moreover, the developmental programs that lead to the early organization of the Z-band are only beginning to be clarified.The study of insect Z-bands has been carried out primarily on the flight muscles of the honeybee (Apis) and the giant water bug (Lethocerus). These insects are particularly favorable for biochemical studies of muscle because their size and the predominance of the flight muscle permit isolation of reasonable amounts of homogeneous muscle tissue. The much smaller size of Drosophila presents obstacles for biochemical analyses but facilitates the genetic analyses that are proving to be another useful approach to the study of muscle structure ...

Abstract. The indirect flight muscles of Drosophila are adapted for rapid oscillatory movements which depend on properties of the contractile apparatus itself. Flight muscles are stretch activated and the frequency of contraction in these muscles is independent of the rate of nerve impulses. Little is known about the molecular basis of these adaptations. We now report a novel protein that is found only in flight muscles and has, therefore, been named flightin. Although we detect only one gene (in polytene region 76D) for flightin, this protein has several isoforms (relative gel mobilities, 27-30 kD; pls, 4.6-6.0). These isoforms appear to be created by posttranslational modifications. A subset of these isoforms is absent in newly emerged adults but appears when the adult develops the ability to fly. In intact muscles flightin is associated with the A band of the sarcomere, where evidence suggests it interacts with the myosin filaments. Computer database searches do not reveal extensive similarity to any known protein. However, the NH2-terminal 12 residues show similarity to the NH2-terminal sequence of actin, a region that interacts with myosin. These features suggest a role for flightin in the regulation of contraction, possibly by modulating actin-myosin interaction.T HE indirect flight muscles (IFM) ~ of Drosophila melanogaster are a group of thoracic fibers whose oscillatory contractions power wing beats at high frequencies, far greater than the firing rate of motor nerves. With steady neural input intracellular calcium levels remain high in the IFM, and the muscle responds to slight changes in length with delayed changes in tension (35,51). This property, called stretch activation, allows the IFM to drive the mechanically resonant wing/thorax system at its very high natural frequency. Since skinned fibers also do oscillatory work in the presence of calcium and ATE it is assumed that components of the myofibril itself are responsible for this adaptation (18).The mechanism underlying stretch activation is not known. Wray (64) has suggested that a slight longitudinal displacement of thick and thin filaments increases the number of cross-bridges recruited by optimally aligning the matching actin and myosin filament periodicities. Recently, however, Squire (45) has challenged this hypothesis on the basis of detailed analysis of filament organization. Alternative proposals are that longitudinal displacement of filaments might influence tension development by changing the angle of attachment of cross-bridges bound to actin (45, 51, 52), J. Vigoreaux's present address is Department of Zoology, University of Vermont, Burlington, VT 05405-0086.

HeT DNA is a complex family of repeated DNA found only in pericentric and telomeric heterochromatin. In contrast to other DNA families that have been specifically associated with heterochromatin, HeT DNA is not principally a family of tandemly repeated elements. Much of the HeT DNA family appears to be a mosaic of several different classes of large sequence elements arranged in a scrambled array; however, some elements of the family can be found in tandem repeats. In spite of the variable order of the different elements in HeT DNA, the sequence homology between different members of each class of element is extremely high, suggesting that the members are evolving in a concerted fashion. Sequence analysis suggests that some elements in the HeT family may make up a novel family of heterochromatin-specific transposable elements and that the mosaic organization of the elements may be produced by retroposition and other mechanisms involved in the transposition of mobile elements. We suggest that such mechanisms may be a general feature for the maintenance of chromosome structure.