Eukaryotic RNA polymerases

A DNA-dependent RNA synthesis by wheat-germ RNA polymerase II insensitive to the fungal toxin alpha-amanitin

Wheat-germ RNA polymerase II is able to catalyse a DNA-dependent reaction of RNA synthesis in the presence of a high concentration (1 mg/ml) of the fungal toxin alpha-amanitin. This anomalous reaction is specifically directed by single-stranded or double-stranded homopolymer templates, such as poly(dC) or poly(dC).poly(dG), and occurs in the presence of either Mn2+ or Mg2+ as the bivalent metal cofactor. In contrast, the transcription of other synthetic templates, such as poly(dT), poly(dA).poly(dT) or poly[d(A-T)] is completely abolished in the presence of 1 microgram of alpha-amanitin/ml, in agreement with well-established biochemical properties of class II RNA polymerases. Size analysis of reaction products resulting from transcription of (dC)n templates of defined lengths suggests that polymerization of RNA chains proceeds through a slippage mechanism. The fact that alpha-amanitin does not impede this synthetic reaction implies that the amatoxin interferes with the translocation of wheat-germ RNA polymerase II along the DNA template.

DNA primase and the replication of the telomeres in Oxytricha nova

An enzymatic activity in crude extracts of macronuclei from the hypotrichous ciliate Oxytricha nova catalyzes the synthesis of RNA consisting of (C4A4)n using an oligodeoxynucleotide template of the telomeric sequence (dG4T4)n. Single-stranded (dG4T4)n is an effective template if it has a random sequence at its 5' end. The enzyme will not use a (dG4T4)n template of any length (up to 64 bases) if it lacks a random sequence at the 5' end. With a random, single-stranded sequence at the 5' end, the (dG4T4)n oligodeoxynucleotide must be at least 36 bases long to work as a template. A 16-base, single-stranded region of (dG4T4)2 is an effective template when joined to a 20-base double-stranded region of (dG4T4)n/(dA4dC4)n, a structural arrangement that is the same as the native telomere of Oxytricha macronuclear DNA. The RNA-synthesizing activity is unaffected by 1.0 mg/ml of alpha-amanitin. Macronuclear extracts have an alpha-amanitin-insensitive, RNA-polymerizing activity that can use a random 55mer oligodeoxynucleotide as a template. This enzyme activity may be the same one that uses (dG4T4)n templates to make (C4A4)n RNA. The (C4A4)n RNA made in the reaction can prime DNA synthesis by the E. coli DNA polymerase I Klenow fragment. Therefore, the RNA polymerase activity fulfills the requirements of the telomere DNA primase that we postulated for replication of telomeres in hypotrichs (Zahler and Prescott, 1988, Nucleic Acids Research 16, 6953-6972).