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Ribonucleic acid (RNA) from encephalomyocarditis (EMC) virus stimulates the incorporation of amino acids into protein in cell-free protein-synthetic systems derived from Krebs mouse ascites tumor cells and chick embryo fibroblasts; the mouse system is the more responsive to the viral RNA. The greater part of this difference in activity can be ascribed to the cell sap, but the origin of the ribosomes also has a marked effect. The nature of the polypeptides formed in these cell-free systems was investigated by electrophoresis on polyacrylamide gels and by fingerprint analysis of tryptic digests. The same product in part appears to be synthesized in response to the EMC RNA in both systems. It was not detected if the EMC RNA was partly degraded (<4S) or replaced by other species of RNA, including that from influenza virus. The results suggest that EMC RNA is partially translated in these systems to yield virus-specific polypeptides.

A light-activated GTPase that functions as a component of the rhodopsin-linked, light-activated phosphodiesterase (PDEase) system in vertebrate photoreceptors has been reported. In our efforts to purify photoreceptor GTPase we encountered another component (which we call "helper" or "H" component) whose presence is required for expression of light-activated GTPase activity. We report here the characterization of this heat-labile, macromolecular factor and that the presence of helper is absolutely required for light-and rhodopsin-dependent activation of photoreceptor GTPase. Of equal importance, we find that the "G" component (which requires the presence of H for expression of GTPase activity) can bind GTP and can support light-and GTP-dependent PDEase activation in the absence of H component. These data support a model in which GTP binding to G component is a necessary condition for PDEase activation. Hydrolysis of GTP at the G activator locus (an H-dependent activity) is a regulatory event which reverses PDEase activation. The complexity of this regulatory mechanism provides opportunities for signal modulation and amplification.

Extensive studies on cells of animal origin from a wide variety of tissues have already established the localization of enzymic activity in cytoplasmic particles. The enzymic activities of mitochondria from animal tissues indicate that these structural units are sites of respiratory activity (see . A more limited series of studies have shown that the mitochondria from plant cells have a similar function Goddard & Stafford, 1954). However, in neither plant nor animal cells is there as yet any clear indication of the metabolic function of the submicroscopic cytoplasmic particles (so-called 'microsomes'), although a number of enzymic activities appear to

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Keilin (1925) first described the cytochromes in a wide variety of whole tissues from animals and plants. Yakushiji (1935) and Okunuki (1939) later studied the distribution of several cytochromes in higher plants, and, in particular, in algae, from which Yakushiji (1935) extracted cytochrome c. Mann (1938) found that the haematin of nonphotosynthetic plant tissues is largely concentrated in the meristematic zones. From the disrupted