Febs Letters Volume 125, Number 1-2./Volume 126, Number 1-2. [antikvár]

Volume 125, number 1 FEBS LETTERS March 1981 Review Letter PROCESSING OF BACTERIAL RNA David APIRION and Peter GEGENHEIMER Department of Micfobiology and Immunology, Washington University School of Medicine, Box 8093, St Louis, MO 63110, USA Received 5 January 1981 1. Introduction Processing of RNA is a feature of RNA metabolism which contributes to the determination of the final population of active RNA molecules in the cell. Processing is the sum of events which converts a primary RNA transcript into a functional molecule. RNA processing in bacteria consists mainly of two parts, modification of nucleotides and cutting and trimming of the molecule; here, we shall concentrate only on the latter. There is ample evidence that rRNA and tRNA go through extensive processing events in the bacterial cell (this area is covered by a number of articles in recent symposia [1,2]). During the processing events, which occur mainly during transcription [3], specific endoribonucleases introduce cuts into the growing transcripts. At present at least three such nucleases are known in Escherichia coll. RNase F [4]; RNase III [3,5]; and RNase E [6-8]. (This short review cannot be all-inclusive, and will reflect our own bias. Because of space limitations we shall emphasize only certain aspects of RNA processing and in each area we shall be able to mention only some of the references.) Mutants which affect these three enzymes are available and strains have been constructed which are defective in one, two or three of these enzymes [9]. By studying tRNA and rRNA synthesis at temperatures permissive and non-permissive for these strains, it was deduced that these three enzymes are the major participants in the primary processing (processing which occurs during transcription) of tRNA and rRNA molecules in the cell and that there is probably one more enzyme which participates in endonu-cleolytic primary processing of rRNA and tRNA [10]. While primary processing of rRNA yields in E. coli the immediate precursors of the mature rRNA species, it is the secondary processing events which yield the final mature RNAs: 16 S, 23 S and 5 S. At present, while the gross features of primary rRNA processing are emerging, knowledge about secondary RNA processing is very minimal. However, there is one essential feature which seems to distinguish these two processes, i.e., namely the requirement for proteins. While all steps of RNA processing occur in the cell at the level of ribonucleoprotein particles, there does not seem to be a need for any protein beside the RNA processing enzymes in primary processing, whereas in secondary processing the substrate seems to be obligatorily an RNP particle [11—13]. The overall process of primary and secondary rRNA processing in E. coli is depicted in fig.l. 2. Enzymes involved in RNA processing A large number of enzymes has been implicated in RNA processing. However, of these only five are sufficiently characterized to be dealt with here. These are RNase P [14,15], RNase III [5,16], RNase E [8], RNase D [17] and RNase M5 [18]. From analysis of tRNA precursors accumulated in RNase P" mutants, it is evident that RNase P is required for 5'-maturation of most E. coli tRNA species [15]. RNase P is an interesting processing enzyme since its enzymatic activity requires both a polypeptide and an RNA component [19,20]. Since no primary sequence similarities exist among all the numerous RNA molecules which are substrates for RNase P, it is apparent that secondary structure, and tertiary conformation play a major role in determining the specificity of RNase P. The correct conformation of the mature portion of a tRNA precursor is a part of the recognition signal [15,21]. RNase III was first purified as an endonucleolytic activity which converted double-stranded RNA to Elsevier/North-Holland Biomedical Press 1