Incubation of excised Avena leaves in a wet chamber in darkness resulted in an increase in both soluble and particle‐bound Rnase activities. Illumination promoted the increase in the total RNase which occurred upon leaf excision. The light‐induced increase in total RNase was due to an increase in soluble RNase. The increase in RNase activity in the particulate fraction was inhibited by illumination. Feeding 2 per cent sucrose to the tissues in the dark increased the level of soluble RNase and decreased the activity found in the particulate fraction. Treatment of the illuminated tissues with 10−4M dichlorophenyldimethylurea (DCMU) inhibited the effects of light on the RNase level. It is concluded that the light‐effect is explained at least in part by the photosynthetic production of sugars. In excised leaves kept in darkness the RNA content rapidly decreased. Feeding sugars to or illumination of the tissues lowered the rate of RNA breakdown due to leaf excision. DCMU counteracted the light effect. In general, the decrease of RNA was repressed by all treatments leading to an inhibition of the increase of particulate RNase. On the other hand, the observed changes of the soluble RNase were not related with the variations of RNA. Treatment with 3 M urea increased the RNase activity both in the particulate and the soluble fractions. The RNase activity of soluble preparations, partially purified on a Sephadex G‐50 column or by (NH4)2SO4 fractionation, was also stimulated by 3 M urea. Treatment with 10−5M kinetin repressed the increase in RNase activity due to leaf excision both in the soluble and the particulate fractions.

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Glucose-6-phosphate dehydrogenase (G6PDH) was isolated from heterocysts and vegetative cells of Anabaena sp. strain PCC 7120. Both enzyme preparations proved to be more active in their oxidized than in their reduced forms. At least one protein with thioredoxin activity was found in Anabaena sp. which, if reduced with dithiothreitol, deactivated the G6PDH preparations. The deactivated heterocyst G6PDH could be reactivated neither by 02 nor by oxidized thioredoxin. Reactivation of the enzyme was, however, achieved by oxidized glutathione or H202. The active form of Anabaena G6PDH was readily deactivated by heterologous thioredoxin(s). The Anabaena thioredoxin(s) modulated heterologous enzymes.