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SynopsisThe flow dichroism of dilute DNA solutions (AZ60 = 0.1) has been studied in a Couette-type apparatus with the outer cylinder rotating and with the light path parallel to the cylinder axis. Shear gradienw in the range of 5-160 see.-' were studied. The DNA samples were whole, "half," and "quarter" molecules of T4 bacteriophage DNA, and linear and circular Xb2b.c DNA. For the linear molecules, the fractional flow dichroism is a linear function of molecular weight. DNA is about 1.8 that of the circular molecule. For a given DNA, the dichroism is an approximately linear function of shear gradient, but with a slight upward curvature at low values of G, and some trend toward saturation at larger values of G. The fractional dichroism increases as the supporting electrolyte concentration decreases. The dichroism for linearIt is well known that native DNA shows strong negative dichroism and flow birefringence in accordance with the expectation for the B form of the Watson-Crick structure in which the planes of the bases are perpendicular to the helix axis. The B form is the common form for DNA fibers a t high humidity and presumably the same molecular structure occurs for native DNA molecules in solution.' Several experimental studies of the flow dichroism of DNA have been made previously. I n one apparatus the observations are made with the light beam propagating perpendicular to a very thin rectangular channel, with the fluid velocity vector along the channel, and with the velocity gradient therefore parallel to the light beam.2-5 This apparatus requires a rather high DNA concentration. A second popular apparatus uses two concentric cylinders with the inner transparent cylinder rotating, and with the light beam perpendicular to the stream lines and to the cylinder axis, that is, again parallel to the velocity This apparatus also requires rather high DNA concentrations because of the short optical paths (1.4 mm.). Furthermore, there is a distinct possibility that the desired, simple laminar flow pattern does not occur in this apparatus at high shear rates because of Taylor instability (see below).We therefore felt that it was desirable to construct a flow dichroism apparatus of the conventional Couette type, with the outer cylinder rotating for flow stability, arid with the light beam along the cylinder axis so that long light paths and low macromolecule concentrations can be used. With this apparatus the basic flow dichroism parameters, especially the 531

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The rosy locus in Drosophila melanogaster codes for the enzyme xanthine dehydrogenase (XDH). Previous studies defined a "control element" near the 5′ end of the gene, where variant sites affected the amount of rosy mRNA and protein produced. We have determined the DNA sequence of this region from both genomic and cDNA clones, and from the ry  +10 underproducer strain. This variant strain had many sequence differences, so that the site of the regulatory change could not be fixed. A mutagenesis was also undertaken to isolate new regulatory mutations. We induced 376 new mutations with 1-ethyl-1-nitrosourea (ENU) and screened them to isolate those that reduced the amount of XDH protein produced, but did not change the properties of the enzyme. Genetic mapping was used to find mutations located near the 5′ end of the gene. DNA from each of seven mutants was cloned and sequenced through the 5′ region. Mutant base changes were identified in all seven; they appear to affect splicing and translation of the rosy mRNA. In a related study (T. P. Keith et al. 1987), the genomic and cDNA sequences are extended through the 3′ end of the gene; the combined sequences define the processing pattern of the rosy transcript and predict the amino acid sequence of XDH.