Acetylation of histones takes place along the salivary gland chromosomes of Chironomus thummi when RNA synthesis is active. It can be observed but not measured quantitatively by autoradiography of chromosome squashes. The "fixatives" commonly used in preparing squashes of insect chromosomes preferentially extract the highly acetylated "arginine-rich" histone fractions; the use of such fixatives may explain the reported absence of histone acetylation in Drosophila melanogaster.

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Both exponentially growing and serum-arrested subcloned CV-1 cell cultures were infected with simian virus 40 (SV40). By 24 h after infection 96% of the nuclei of these permissive cells contained SV40 T-antigen. Analysis of the average DNA content per cell at various times after infection indicated that by 24 h most of the cells contained amounts of DNA similar to those normally found in G 2 cells. Analysis of cell cycle distributions indicated that a G 2 DNA complement was maintained by over 90% of the cells in the infected populations 24 to 48 h postinfection. Cells continued to synthesize SV40 DNA during the first 50 h after infection, and cytopathic effect was first observed 60 h after inoculation. After infection the number of mitotic cells that could be recovered by selective detachment decreased precipitously and was drastically reduced by 24 h. A study of the kinetics of decline in the number of mitotic cells suggests that this decline is related to an event during the cell cycle at or near the G 1 -S-phase border upon which commencement of SV40 DNA replication apparently depends. It was concluded that after SV40 infection, stationary cells are induced to cycle, and cycling cells complete one round of cellular DNA synthesis but do not divide. Although the infected cells continue to synthesize viral DNA, they do not appear able to reinitiate cellular DNA replication units. These results imply that the abundance of T-antigen (produced independently of cell cycle phase) in the presence of the enzymes required for continued DNA synthesis is not sufficient for reinitiation of cellular DNA synthesis.

By adding Teflon tubing to the effluent line of the Ortho 50H analytical cytofluorograph, we were able to increase the sensitivity of fluorescence and scatter detection threefold without compromising resolution. By increasing sheath backpressure (resistance), the additional tubing increases particle residence time within the detection area and thereby increases the total photon emission density per particle. In addition, the longer, For maximum flow cytometer sensitivity and resolution, particles must be analyzed within a quartz flow cell. To achieve this, the sample stream is focussed hydrodynamically through a square flow cell orifice, thereby eliminating the optical aberrations associated with curved surfaces (1,3). High-resolution optics and additional lenses have also been employed to maximize the focussing of scattered and emitted light onto sensing devices (2,4-6). However, given these optical and fluidic considerations, little attention has been paid to the effect of effluent tubing dimensions on analytical flow cell hydrodynamics.Typically, the effluent stream is slowed to a n acceptable velocity by adding approximately one foot of 0.3-mm inner diameter (i.d.1 tubing suspended over a waste reservoir. However, this tubing is often the site of blockages that result in signal-altering turbulence.To correct this, users cut off the clogged ends thereby shortening the coil and, consequently, increasing the flow rate. Eventually, particle residence time becomes too short to produce acceptable signals and the coil is replaced.To assess the effect of effluent tubing dimensions on sensitivity, we measured the integrated green fluorescence and scatter signals of fluorescent polystyrene beads as a function of length of 0.30-mm) 0.59-mm or 0.81-mm i.d. effluent tubing. Our results indicate that both the sensitivity of detection and system stability can be improved by altering effluent tubing length andlor inner diameter.wider tubing dampens the stream-destabilking effects of minor obstructions in or movement of the effluent line. This increase in sensitivity and stability is desirable in applications requiring detection of particles exhibiting low-level fluorescence.

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