Bacterial RNA Polymerase Contains Five Subunits

Escherichia coli contains one RNA polymerase, which transcribes all three major types of RNA. The active enzyme is a pentamer containing four different polypeptide chains with a total molecular weight of about 500,000. The subunits of the enzyme can be separated by electrophoresis on polyacrylamide gels. The four different polypeptide chains, termed j8, /3, a10, and a, have molecular weights of 155,000, 151,000, 70,000, and 36,500, respectively. Additional cr-like proteins have been identified, which we discuss later. 

A complex with the subunit structure a2PP cr70 can carry out the functions necessary for synthesis of RNA and is referred to as the holoenzyme. Holoenzyme can be reversibly separated into two components by chromatography on a phosphocellulose column. 

Arrangement of components in the E. coli 30S ribosomal particle. In (a) the relative locations of the ribosomal proteins, numbered 1-21, are shown. (Illustration prepared by Dr. Malcolm Capel from data described in M. S. Capel, M. Kjeldguard, D. M. Engelman,. /. Mol. Biol. 200 66-87, 

The enzyme without the sigma factor, called core polymerase, retains the capability to synthesize RNA, but it is defective in the ability to bind and initiate transcription at true initiation sites on the DNA. In fact when RNA polymerase was first purified from crude extracts it was missing the a factor. The assay for polymerase involved the use of DNA with single-strand nicks. When a DNA template was used that did not have single-strand nicks, this enzyme was not active. This led to a search for a missing factor. When this factor (cr70) was added back to the purified core enzyme and the uncut DNA template, the enzyme was able to bind 

) In (b) the conformation of the rRNA is shown. The location of the proteins is indicated by numbers given in the figure. (Illustration prepared by S. Stern, B. Weiser, and H. F. Noller from data described in J. Mol. Biol. 204 447-481, 1988.) 

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Bacterial RNA Polymerase Contains Five Subunits Binding at Promoters Initiation at Promoters... 

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