8-azaguanine and flavinogenesis in Eremothecium ashbyii.

8-Azaguanine (10- minus 4 M) supplementation in synthetic medium inhibited flavinogenesis in Eremothecium ashbyii to far greater extent (68per cent) than the growth (25 per cent). That enzymes comprising the biosynthetic pathway of riboflavin are synthesized during early growth phase of the organism is supported by the data presented. 8-Azaguanine mediated inhibition in flavinogenesis was closely related with decreased levels of ribose-5'-phosphatase, ribose reductase and ribitol kinase, the enzymes involved in supplying ribitol for flavinogenesis. Addition of guanine and not ribitol during early growth phase to 8-azaguanine-added cultures released the inhibition of riboflavin synthesis and restored the enzyme levels in the presence of the antimetabolite.     

STUDIES ON THE ORIGIN OF PYRIMIDINES FOR BIOSYNTHESIS OF NEURAL RNA IN THE RAT

Uridine was far superior to orotic acid in labelling the RNA in incubated slices of rat brain. On the other hand, uridine and orotic acid were equally effective in labelling the RNA of hepatic or renal slices In rats in vivo, uridine, but not orotic acid, labelled brain RNA, and the cerebellar RNA contained the most label. In contrast, both uridine and orotic acid labelled hepatic RNA. Only when surgical intervention prevented peripheral metabolism of orotic acid, thereby raising its concentration in the plasma, did neural tissue utilize this precursor for limited biosynthesis of RNA. However, among the tissues studied, the preference for uridine over orotic acid for RNA synthesis was unique to neural tissue.     

STUDIES ON THE BIOSYNTHESIS OF COLLAGEN : I. THE GROWTH OF FOWL OSTEOBLASTS AND THE FORMATION OF COLLAGEN IN TISSUE CULTURE

1. A tissue culture method was devised in which suspensions of osteoblasts, obtained directly from frontal bones of fowl embryos, were grown in a fluid, fibrin-free medium. 2. Maximum growth of the tissue, as measured by dry weight, with the formation of collagen protein, based on the estimation of hydroxyproline, was obtained in periods of up to 6 days. 3. Appreciable amounts of protein-bound hydroxyproline were formed during the first 24 hour growth period, but electron microscopy of portions of the same cultures failed to demonstrate the presence of any typical collagen fibrils. 4. The subsequent formation of many characteristic collagen fibrils was not associated with a significant rise in the mean hydroxyproline content of the tissue. 5. The cytoplasmic granules of the osteoblasts stained intensely with the P.A.S. technique when the collagen fibrils were being formed. 6. It is suggested that collagen-forming cells synthesise and secrete a hydroxyproline-rich precursor of protein or large peptide nature, which subsequently becomes directly transformed into typical collagen fibrils.