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Journal of Bacteriology, August 2003, p. 4382-4392, Vol. 185, No. 15
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.15.4382-4392.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

immX Immunity Region of Rhizobium Phage 16-3: Two Overlapping Cistrons of Repressor Function

Institute of Genetics, Agricultural Biotechnology Center, Gödöllõ, H-2100,¹ Department of Genetics, Faculty of Science, Eötvös Lóránd University, Budapest, H-1117, Hungary,² Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892-4264³

Received 3 February 2003/ Accepted 1 May 2003

16-3 is a temperate phage of the symbiotic nitrogen-fixing bacterium Rhizobium meliloti 41. Its prophage state and immunity against superinfection by homoimmune phages are governed by a complex set of controls: the immC and immX repressor systems and the avirT element are all located in well-separated, distinct regions which span 25 kb on the bacteriophage chromosome. The anatomy and function of the immC region are well documented; however, fewer analyses have addressed the immX and avirT regions. We focused in this paper on the immX region and dissected it into two major parts: X_U/L and X_V. The X_U/L part (0.6 kb) contained two overlapping cistrons, X_U and X_L, coding for proteins pXU and pXL, respectively. Inactivation of either gene inactivated the repressor function of the immX region. Loss-of-function mutants of X_U and X_L complemented each other in trans in double lysogens. The X_V part (1 kb) contained a target for X_U/L repressor action. Mutations at three sites in X_V led to various degree of ImmX insensitivity in a hierarchic manner. Two sites (X_V1 and X_V3) exhibited the inverted-repeat structures characteristic of many repressor binding sites. However, X_V1 could also be folded into a transcription terminator. Of the two immunity regions of 16-3, immX seems to be unique both in its complex genetic anatomy and in its sequence. To date, no DNA or peptide sequence homologous to that of ImmX has been found in the data banks. In contrast, immC shares properties of a number of immunity systems commonly found in temperate phages.

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