PRIMARY STRUCTURE REQUIREMENTS FOR IN-VIVO ACTIVITY AND BIDIRECTIONALFUNCTION OF THE TRANSCRIPTION TERMINATOR SHARED BY THE OPPOSITELY ORIENTED SKC REL-ORF1 GENES OF STREPTOCOCCUS-EQUISIMILIS H46A/

The region between the Streptococcus equisimilis streptokinase (skc) g ene and the oppositely oriented rel-orf1 transcription unit contains o nly one termination site known to function bidirectionally in both the homologous host and in Escherichia coli. The terminator sequence is s imilar to other factor-independent terminators. Using two sets of poin t mutations that interrupt the hairpin-upstream oligo(dA) tract or the hairpin-downstream oligo(dT) tract, we examined the possible contribu tion of extended base pairing between the upstream rA and downstream r U residues to efficient termination and bidirectionality in both hosts , using terminator-cat reporter gene fusions in either polarity. The r esults show that interrupting the oligo(dA:) tract preceding the hairp in has relatively little effect on terminator strength in either orien tation in the homologous host, but abolishes termination in skc polari ty in E. coli. Disruption of the hairpin-distal oligo(dT) tract inacti vated the terminator in skc polarity in both hosts, had little effect on termination efficiency in rel-orf1 polarity in S. equisimilis, and also retained appreciable terminator activity in E. coli. In general, these alterations of the terminator sequence, together with additional mutations that reduce the spacing between the site stop codon and the termination site or introduce a base substitution in the terminator s tem, adversely affected the efficiency of termination to a greater ext ent in E. coli than in the homologous host. The disparity between the effects of certain mutations in the two hosts suggests that, in additi on to thermodynamic properties, specific host factors, including RNA p olymerase, contribute to terminator strength.