ESCHERICHIA-COLI PROTEIN ANALOGS STPA AND H-NS - REGULATORY LOOPS, SIMILAR AND DISPARATE EFFECTS ON NUCLEIC-ACID DYNAMICS

Expression of the Escherichia coil StpA protein was investigated and a functional comparison undertaken with the structurally analogous nucl eoid protein H-NS. Analysis of stpA and hns expression indicated that although stpA transcript levels are much lower than those of hns, the two gene products are capable of both negative autogenous control and cross-regulation. Examination of cellular proteins in stpA, hns, or st pA-hns backgrounds revealed that StpA can repress and activate a subse t of H-NS-regulated genes. Mechanistic parallels in regulation of gene expression are indicated by the ability of both proteins to inhibit t ranscription from promoters containing curved DNA sequences, and to fo rm nucleoprotein structures that constrain DNA supercoils. Despite the ir functional similarities, each molecule is capable of independent ac tivities. Thus, H-NS regulates a class of genes that are unaffected by StpA in vivo, whereas StpA has much stronger RNA chaperone activity i n vitro. We therefore propose that in addition to its role as a molecu lar back-up of H-NS, StpA's superior effect on RNA may be exploited un der some specific cellular conditions to promote differential gene exp ression.