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The cr subunit confers promoter-specific transcription initiation on RNA polymerase (4; for reviews, see references 21 and 24). The identification, characterization, and sequence analysis of many sigma factors have revealed that they fall into two broad classes, with little if any sequence identity between them (37, 41). One family is similar to the originally identified Escherichia coli ur70 subunit; the other is similar to the 54-kDa E. coli cr subunit (for reviews, see references 32 and 52). Recently, sequence information has become avail-able for many more members of the u70 family. In addition, a great deal has been learned about the functions of various subregions of c70. In this minireview, we present an updated alignment of the Cr70 family of proteins and summarize the current understanding of structure, function, and evolution-ary relationships of this important family of proteins.

SEQUENCE ALIGNMENT AND …