A novel transcription initiation factor (TIF), TIF-IE, is required for homogeneous Acanthamoeba castellanii TIF-IB (SL1) to form a committed complex.

The fundamental transcription initiation factor (TIF) for ribosomal RNA expression by eukaryotic RNA polymerase I, TIF-IB, has been purified to near homogeneity from Acanthamoeba castellanii using standard techniques. The purified factor consists of the TATA-binding protein and four TATA-binding protein-associated factors with relative molecular weights of 145,000, 99,000, 96,000, and 91,000. This yields a calculated native molecular weight of 460, 000, which compares well with its mass determined by scanning transmission electron microscopy (493,000) and its sedimentation rate, which is close to RNA polymerase I (515,000).

Acanthamoeba castellanii contains a ribosomal RNA enhancer binding protein which stimulates TIF-IB binding and transcription under stringent conditions.

The intergenic spacer (IGS) of Acanthamoeba castellanii rRNA genes contains repeated elements which are weak enhancers for transcription by RNA polymerase I. A protein, EBF, was identified and partially purified which binds to the enhancers and to several other sequences within the IGS, but not to other DNA fragments, including the rRNA core promoter. No consensus binding sequence could be discerned in these fragments and bound factor is in rapid equilibrium with unbound.

Initiation and regulation mechanisms of ribosomal RNA transcription in the eukaryote Acanthamoeba castellanii.

Acanthamoeba rRNA transcription involves the binding of a transcription initiation factor (TIF) to the core promoter of rDNA to form the preinitiation complex. This complex is formed in the absence of RNA polymerase I, and persists for multiple rounds of initiation. Polymerase I next binds to form the initiation complex.

Regulation of bile acid synthesis. I. Effects of conjugated ursodeoxycholate and cholate on bile acid synthesis in chronic bile fistula rat.

Bile acid synthesis is thought to be regulated by a negative feedback mechanism which is presumably dependent upon the flux of bile acids in the enterohepatic circulation. To characterize further the role of bile acids in regulation of bile acid synthesis, we have administered pure taurine or glycine conjugates of ursodeoxycholic acid or cholic acid to chronic bile fistula rats by continuous intraduodenal infusion, thus simulating restoration of the enterohepatic circulation. The effects of these bile salt infusions on bile acid synthesis, biliary cholesterol and phospholipid secretion and on the activities of the hepatic microsomal enzymes cholesterol 7 alpha-hydroxylase and HMG-CoA reductase were evaluated.

Effects of ursodeoxycholic acid, analogues of ursodeoxycholic acid and combination of bile acids on bile acid synthesis in cultured rat hepatocytes.

The effect of individual 7 beta-hydroxy bile acids (ursodeoxycholic and ursocholic acid), bile acid analogues of ursodeoxycholic acid, combination of bile acids (taurochenodeoxycholate and taurocholate), and mixtures of bile acids, phospholipids and cholesterol in proportions found in rat bile, on bile acids synthesis was studied in cultured rat hepatocytes. Individual steroids tested included ursodeoxycholate (UDCA), ursocholate (UCA), glycoursodeoxycholate (GUDCA) and tauroursodeoxycholate (TUDCA). Analogues of UDCA (7-methylursodeoxycholate, sarcosylursodeoxycholate and ursooxazoline) and allochenodeoxycholate, a representative of 5 alpha-cholanoic bile acid were also tested in order to determine the specificity of the bile acid biofeedback.