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J. Biol. Chem., Vol. 277, Issue 9, 7262-7270, March 1, 2002
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§,¶,
, and
From the Departamento de Genética y Microbiología,
Facultad de Biología, Universidad de Murcia,
Murcia 30071, Spain
The light-inducible carB operon
encodes all but one of the structural genes for carotenogenesis in
Myxococcus xanthus. It is transcriptionally controlled by
two proteins expressed from two unlinked genetic loci: CarS from the
light-inducible carQRS operon, and CarA from the
light-independent carA operon. CarA represses transcription
from the carB promoter (PB) in the dark, and
CarS counteracts this on illumination. The CarA sequence revealed a
helix-turn-helix DNA-binding motif of the type found in bacterial MerR
transcriptional factors, whereas CarS contains no known DNA-binding motif. Here, we examine the molecular interplay between CarA and CarS.
We demonstrate the following. (i) Whereas CarS exhibits no DNA binding
in vitro, CarA binds specifically to a region encompassing PB to form at least two distinct complexes. (ii) A
palindrome located between positions 
46 and 63 relative to the
transcription start point is essential but not sufficient for the
formation of the two CarA-DNA complexes observed. (iii) CarS abrogates
the specific DNA binding of CarA. CarA is therefore a repressor and CarS an antirepressor. (iv) CarS physically interacts with CarA; thus,
the functional interaction between them is mediated by protein-protein interactions.
Both authors contributed equally to this work.
§
Supported by a fellowship from Fundación Séneca
(Murcia, Spain).
¶
To whom correspondence may be addressed. Tel.: 34-968-367-134;
Fax: 34-968-363-963; E-mail: melias@um.es.
Supported by Ministerio de Ciencia y Tecnología, Spain.
**
To whom correspondence may be addressed. Tel.: 34-968-364-951; Fax:
34-968-363-963; E-mail: araujo@um.es.
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