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Bacteriophage P4 sut1: a mutation suppressing transcription termination

Susanna Terzano

Ilaria Oliva

Dipartimento di Scienze Biomolecolari e Biotecnologie, Università degli Studi di Milano, Via Celoria 26, 20133 Milano, Italy

Correspondence
Daniela Ghisotti
Daniela.Ghisotti{at}unimi.it

In the Escherichia coli satellite phage P4, transcription starting from P_LE is prevalently controlled via premature termination at several termination sites. We identified a spontaneous mutation, P4 sut1 (suppression of termination), in the natural stop codon of P4 orf151 that, by elongating translation, suppresses transcription termination at the downstream t₁₅₁ site. Both the translational and the transcriptional profile of P4 sut1 differed from those of P4 wild-type. First of all, P4 sut1 did not express Orf151, but a higher molecular mass protein, compatible with the 303 codon open reading frame generated by the fusion of orf151, cnr and the intervening 138 nt. Moreover, after infection of E. coli, the mutant expressed a very low amount of the 1.3 and 1.7 kb transcripts originating at P_LE and P_LL promoters, respectively, and terminating at the intracistronic t₁₅₁ site, whereas correspondingly higher amounts of the 4.1 and 4.5 kb RNAs arising from the same promoters and covering the entire operon were detected. Thus the sut1 mutation converts a natural stop codon into a sense codon, suppresses a natural intracistronic termination site and leads to overexpression of the downstream cnr and genes. This correlates with the inability of P4 sut1 to propagate in the plasmid state. By cloning different P4 DNA fragments, we mapped the t₁₅₁ transcription termination site within the 7633–7361 region between orf151 and gene cnr. A potential stem–loop structure, resembling the structure of a Rho-independent termination site, was predicted by mfold sequence analysis at 7414–7385.

Present address: Molecular Haematology and Cancer Biology Unit, The Institute of Child Health, 30 Guilford Street, London WC1N 1EH, UK.

Present address: OPIS srl, Via Matteotti 10, 20033 Desio (Mi), Italy.