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The complex terminator region of the Escherichia coli rrnB gene was analyzed by subcloning the terminators T1 and T2 and the inverted repeats IR1 and IR2 individually, or in various combinations, in a normal or inverted orientation into a terminator probe vector. The in vivo terminating efficiency was assayed by measuring the galactokinase activity encoded by the downstream galK gene. Termination efficiencies of all fragments were compared in two constructs, differing in the presence or absence of readthrough translation over the investigated terminator signal. The following main conclusions were drawn. The seven genes coding for ribosomal RNA in Escherichia coli have basically similar structures. The ends of six genes have been sequenced [I -61 and are all factor-independent (type I) [7] transcriptional terminators. A characteristic feature of these very large untranslated transcription units is that one or more antitermination regions ensure the complete uninterrupted transcription of the whole operon [8,9]. Several lines of evidence indicates that antiterminated transcriptional complexes of rrn genes are able to transcribe some type I terminator signals [lo]. The experimental evidence is conflicting on the problem as to how the rrn gene terminator regions efficiently stop antiterminated transcription [ll].The rrnB gene is probably unique among the rrn genes because its termination region is large and complex. It contains two regions (TI and T2) with typical factor-independent terminator-like sequences, two additional inverted repeats (lR1 and IR2) and a pair of direct repeats [l]. The whole region is frequently employed as an efficient terminator of transcription in several cloning vectors [12].In this paper we report the detailed molecular analysis of the terminating properties of this region. In particular, we attempted to answer the following questions. (a) What are the terminating efficiencies of the individual subregions of the complex rrnB terminator region? (b) How efficient are the terminators when present in an inverted orientation? (c) What is the effect of translation on the terminating efficiencies of the terminators?Correspondence to P. Venetianer, Institute

The genes of the 5'-GGCC specific BsuRI restriction-modification system of Bacillus subtilis have been cloned and expressed in E. coli and their nucleotide sequence has been determined. The restriction and modification genes code for polypeptides with calculated molecular weights of 66,314 and 49,642, respectively. Both enzymes are coded by the same DNA strand. The restriction gene is upstream of the methylase gene and the coding regions are separated by 780 bp. Analysis of the RNA transcripts by S1-nuclease mapping indicates that the restriction and modification genes are transcribed from different promoters. Comparison of the amino acid sequences revealed no homology between the BsuRI restriction and modification enzymes. There are, however, regions of homology between the BsuRI methylase and two other GGCC specific modification enzymes, the BspRI and SPR methylases.