Studies of the Ribonucleic Acid Polymerase from Escherichia coli

Heterogeneous initiation due to slippage at the bacteriophage 82 late gene promoter in vitro

RNAs synthesized in vitro by purified Escherichia coli RNA polymerase from a bacteriophage 82 promoter are heterogeneous at the 5' end. We show that this heterogeneity results from variable addition of extra adenine residues, allowed by slippage of the initial oligonucleotide pppAAA-OH against its DNA template sequence TTT. Slippage backward by one base allows another A to be added, giving pppAAAA-OH, and this cycle can continue more than 20 times before it is ended by incorporation of UMP encoded by the fourth template base A. Slippage is abolished by mutation of the TTT template sequence to TGT and is sensitive to the concentrations of UTP and ATP in the reaction mixture. Analysis of deletions, substitutions, and point mutants implies that the slippage reaction requires only the existence of TTT at the initiation site of the template strand, although changes in neighboring nucleotides slightly affect its efficiency.

Poly(A) polymerase activity in reovirus

An enzymatic activity which synthesized oligo(A) in vitro was found in highly purified reovirus. The poly(A) polymerase activity was dependent on Mn(2+) and utilized only ATP, whereas the virion-associated RNA polymerase required all four ribonucleoside triphosphates and Mg(2+). Oligo(A) synthesis was demonstrated with complete virions and infectious subviral particles derived from virus by limited chymotrypsin digestion but not with cores, a product of extensive chymotrypsin digestion of virus. The enzymatic product and the oligo(A) from purified virions were isolated by binding to oligo(dT)-cellulose columns. Most of the in vitro product was similar in size and structure to the oligo(A) from purified virions by the criteria of gel electrophoresis, DEAE-cellulose chromatography, end-group analysis, and sensitivity to RNase. The evidence suggests that oligo(A) synthesis is mediated by the poly(A) polymerase during a late step in viral morphogenesis and may result from an alternative activity of the virion-associated transcriptase.