N. Franceschini

A photostable pigment within the rhabdomere of fly photoreceptors no. 7

SummaryThe population of the centrally located rhabdomeres no. 7 in the ommatidia of flies (Musca, Calliphora, Drosophila) is inhomogeneous: approximately 2/3 of them contain — besides a photoisomerizable rhodopsin — a photostable pigment. Its extinction spectrum has a maximum at 460 nm and two shoulders at 430 and 485 nm respectively. Extinction is maximal for theE-vector perpendicular to the microvilli. Whereas the functional role of the photostable pigment for receptor 7 has still to be worked out, its functional consequence for receptors no. 8, which are located proximal to receptors 7, is obvious: it modifies their spectral sensitivity by selectively absorbing blue light. Due to this “screening”-effect, a shift of the maximal sensitivity of receptors no. 8 is predicted from 490 nm (maximal sensitivity of unscreened receptor 8, Harris et al., 1976) to 520 to 540 nm. This is in agreement with recent electrophysiological data (Hardie, 1977). The results show that spectral sensitivities of receptors no. 8, as determined by means of the ERG of white-eyed mutants or of mutants lacking receptor 7, do not represent the spectral sensitivities of most of these receptors in wild-type flies.

Distribution and properties of sex-specific photoreceptors in the flyMusca domestica

Summary1.In male houseflies (Musca domestica) the frontal dorsal region of the eye contains a unique class of central rhabdomere (R7/8) not found in other eye regions or in female flies (Fig. 1). The rhabdomeres may be recognised in vivo by their red autofluorescence, and are called here 7r and 8r respectively.2.Difference spectra of 7r rhabdomeres, measured by microspectrophotometry of single rhabdomeres are indistinguishable from those of R1–6 (Fig. 2).3.Intracellular recordings coupled with dye injections have established that: a) 7r cells are indistinguishable from the peripheral photoreceptors R1–6, at least with respect to spectral, angular and absolute sensitivities, response waveform and noise characteristics (Figs. 4, 5; Table 1); b) 8r cells however are clearly distinguishable by virtue of their spectral sensitivity (Fig. 6), noise characteristics and response waveform (Fig. 5).4.Axonal profiles from cells stained intracellularly with the dye Lucifer yellow (Fig. 9) show that: a) 7r cells do not project to the medulla but terminate in the upper region of the lamina cartridge layer where they also project out one or more lateral branches; b) 8r cells project long axons through to the medulla.5.Electron microscopic examinations of cells initially identified by their autofluorescence indicate that 7r cells approximate many features of R1–6 cells, including cell body, rhabdomere and axonal diameters. In these respects 8r cells differ and show the characteristic morphology of other R7 and R8 cells (Fig. 8, Table 2).

Le contrôle automatique du flux lumineux dans l'oeil composé des Diptères

In the compound eye of the fly Musca, tiny pigment granules move within the cytoplasm of receptor cells Nos. 1–6 and cluster along the wall of the rhabdomeres under light adaptation, thus attenuating the light flux to which the visual pigment is exposed (Kirschfeld and Franceschini, 1969). Two recently developed optical methods (the neutralization of the cornea and the deep pseudopupil) combined with antidromic and orthodromic illumination of the eye (Fig. 1) make it possible to analyse the properties of the mechanism at the level of the single cell, in live and intact insects (Drosophila and Musca). The mechanism is shown to be an efficient attenuator in the spectral range (blue-green) where cells Nos. 1–6 have been reported to be maximally sensitive (Figs. 4c and d, 5b and 11b). In spite of the fact that the granules do not penetrate into the rhabdomere, the attenuation spectrum they bring about closely matches the absorption spectrum of the substance of which they are composed (ommochrome pigment, dotted curve in Fig. 11b). The dramatic increase in reflectance of the receptors after light adaptation (Figs. 3, 4b, 5a and 11a) can be explained as a mere by-product of the high absorption index of the ommochrome pigment, especially if one takes into account the phenomenon of anomalous dispersion (Chapter 8). The vivid green or yellow colour of the rhabdomeres would thus have a physical origin comparable to a metallic glint. Contrasting with the lens eye in which the pupillary mechanism is a common attenuator for both receptor types (rods and cones), the compound eye of higher Diptera is equiped with two types of “pupils” adapted respectively to both visual subsystems. A scotopic pupil is present in each of the six cells (Nos. 1–6) whose signals are gathered in a common cartridge of the first optic ganglion. This pupil comes into play at a moderate luminance (0,3 cd/m2 in Drosophila; 3 to 10 cd/m2 in Musca. Figs 13, 14, 15, 16). A photopic pupil is present in the central cell No. 7 whose signal reaches one column of the second optic ganglion. Attenuating the light flux for both central cells 7 and 8, the photopic pupil has its threshold about two decades higher than the scotopic pupil, just at the point where the latter reaches saturation (Fig. 3b, e-State II of Figs. 6b and 15). The photopic pupil itself saturates at a luminance one to two decades higher still (Fig. 3c, f=State III of Figs. 6c and 15). The two-decades-shift in threshold of these pupil-mechanisms supports the view that receptors 1–6 are a scotopic subsystem, receptors 7 and 8 a photopic subsystem of the dipteran eye. The luminance-threshold of the scotopic pupil (as determined with the apparatus described in Fig. 2) appears to be located at least 3.5 decades (Drosophila) or even 5 decades (Musca) higher than the absolute threshold of movement perception (Fig. 16). After a long period (1 hr) of darkness a light step of high intensity can close the scotopic pupil within about 10 sec (time constant τ≃2 sec as in Fig. 9) and the photopic pupil within no less than 30–60 sec. Some mutants of Drosophila possess only a scotopic pupil (wα, Figs. 4 and 5) whereas ommochrome deficient mutants lack both types of pupil (v, cn, see Fig. 7c, d). Comparable reflectance changes, accomplished within about 60 sec of light adaptation, are described for two insects having fused rhabdomes: the bee and the locust (Fig. 17).

Photostable pigments within the membrane of photoreceptors and their possible role.

In the majority of ommatidia of the fly, the membrane of the central rhabdomere contains — besides the rhodopsin — a photostable pigment. Due to its selective absorption in the blue spectral range, this pigment (possibly a carotene) could modify the spectral sensitivity of the central receptor cells. It furthermore may change the fluidity of the microvillus membrane and hence affect the alignment of rhodopsin molecules. Indirect evidence for a possible role of the photostable pigment as an “antenna”-pigment for rhodopsin is discussed.

Use of the chromosomal class A beta-lactamase of Mycobacterium fortuitum D316 to study potentially poor substrates and inhibitory beta-lactam compounds.

Sixteen different compounds usually considered beta-lactamase stable or representing potential beta-lactam inhibitors and inactivators were tested against the beta-lactamase produced by Mycobacterium fortuitum. The compounds exhibiting the most interesting properties were BRL42715, which was by far the best inactivator, and CGP31608 and ceftazidime, which were not recognized by the enzyme. These compounds thus exhibited adequate properties for fighting mycobacterial infections. Although cloxacillin, dicloxacillin, cefoxitin, and CP65207-2 exhibited poor inhibitory efficiency against the enzyme, they were also rather poor substrates and might be considered potential antimycobacterial agents. By contrast, CGP31523A and ceftamet were good substrates.

FLUORESCENCE OF HOUSEFLY VISUAL PIGMENT

Abstract —The fluorescence of housefly photoreceptors was studied in vivo by using the deep pseudopupil technique. Whereas the rhodopsin R490 of the peripheral retinular cells fluoresces negligibly the metarhodopsin M580 fluoresces distinctly in the red. The newly discovered metarhodopsin M’is produced by intense blue light and can be reconverted into rhodopsin by intense long wavelength light. M’also fluoresces in the red; its excitation spectrum and emission spectrum peak at max= 570 and 660 nm respectively.

Interactions of biapenem with active-site serine and metallo-beta-lactamases.

Biapenem, formerly LJC 10,627 or L-627, a carbapenem antibiotic, was studied in its interactions with 12 beta-lactamases belonging to the four molecular classes proposed by R. P. Ambler (Philos. Trans. R. Soc. Lond. Biol. Sci. 289:321-331, 1980). Kinetic parameters were determined. Biapenem was readily inactivated by metallo-beta-lactamases but behaved as a transient inhibitor of the active-site serine enzymes tested, although with different acylation efficiency values. Class A and class D beta-lactamases were unable to confer in vitro resistance toward this carbapenem antibiotic. Surprisingly, the same situation was found in the case of class B enzymes from Aeromonas hydrophila AE036 and Bacillus cereus 5/B/6 when expressed in Escherichia coli strains.