Global versus local regulatory roles for Lrp-related proteins: Haemophilusinfluenzae as a case study

Lrp (leucine-responsive regulatory protein) plays a global regulatory role in Escherichia call, affecting expression of dozens of operons, Numerous lr p-related genes have been identified in different bacteria and archaea, inc luding asnC, an E, coli gene that was the first reported member of this fam ily. Pairwise comparisons of amino acid sequences of the corresponding prot eins shows an average sequence identity of only 29% for the vast majority o f comparisons. By contrast, Lrp-related proteins from enteric bacteria show more than 97% amino acid identity. Is the global regulatory role associate d with E, coli Lrp limited to enteric bacteria? To probe this question we i nvestigated LrfB, an Lrp-related protein from Haemophilus influence that sh ares 75% sequence identity with E, coli Lrp (highest sequence identity amon g 42 sequences compared), A strain of H. influence having an lrfB null alle le grew at the wild-type growth rate but with a filamentous morphology, A c omparison of two-dimensional (2D) electrophoretic patterns of proteins from parent and mutant strains showed only two differences (comparable studies with Irp(+) and Irp E. coli strains by others showed 20 differences). The a bundance of LrfB in H. influenzae, estimated by Western blotting experiment s, was about 130 dimers per cell (compared to 3,000 dimers per E. coli cell ). LrfB expressed in E. coli replaced Lrp as a repressor of the Irp gene bu t acted only to a limited extent as an activator of the ilvIH operon, Thus, although LrfB resembles Lrp sufficiently to perform some of its functions, its low abundance is consonant with a more local role in regulating but a few genes, a view consistent with the results of the 2D electrophoretic ana lysis, We speculate that an Lrp having a global regulatory role evolved to help enteric bacteria adapt to their ecological niches and that it is unlik ely that Lrp-related proteins in other organisms have a broad regulatory fu nction.