The porA gene encodes the class 1 outer membrane protein (OMP1) in Neisseria meningitidis and is under transcriptional control. Promoter regions of porA from different clinical isolates were sequenced and were found to differ in the number of guanosine residues in a poly(G) track located upstream of the -10 region. Isolates that did not express OMP1 had up to nine G residues in the poly(G) track or an adenosine residue within this poly(G) track. Using beta-galactosidase as a reporter gene, the transcriptional activities of the promoter regions of the porA gene from three strains, two of which do not express OMP1, were assayed in both Escherichia coli and N. meningitidis. Mutations in the poly(G) track were created by site-directed mutagenesis and promoter fusions were further analyzed in E. coli and N. meningitidis. The number of nucleotides in the poly(G) track influenced promoter activity: reduction of a poly(G) track of 12nt by one and by two guanosine residues reduced promoter activity. Within the poly(G) track, replacement of an adenosine residue by a guanosine residue increased the promoter activity; replacement of a guanosine residue by an adenosine residue decreased the activity. The similar transcriptional activities for the mutated promoters in E. coli and N. meningitidis are compatible with similar control mechanisms for transcriptional control in both organisms.