The TolC-like protein HgdD of the filamentous, heterocyst-forming cyanobacterium 
Anabaena sp. PCC 7120 is part of multiple three-component "AB-D" systems 
spanning the inner and outer membranes and is involved in secretion of various 
compounds, including lipids, metabolites, antibiotics, and proteins. Several 
components of HgdD-dependent tripartite transport systems have been identified, 
but the diversity of inner membrane energizing systems is still unknown. Here we 
identified six putative resistance-nodulation-cell division (RND) type factors. 
Four of them are expressed during late exponential and stationary growth phase 
under normal growth conditions, whereas the other two are induced upon 
incubation with erythromycin or ethidium bromide. The constitutively expressed 
RND component Alr4267 has an atypical predicted topology, and a mutant strain 
(I-alr4267) shows a reduction in the content of monogalactosyldiacylglycerol as 
well as an altered filament shape. An insertion mutant of the ethidium 
bromide-induced all7631 did not show any significant phenotypic alteration under 
the conditions tested. Mutants of the constitutively expressed all3143 and 
alr1656 exhibited a Fox(-) phenotype. The phenotype of the insertion mutant 
I-all3143 parallels that of the I-hgdD mutant with respect to antibiotic 
sensitivity, lipid profile, and ethidium efflux. In addition, expression of the 
RND genes all3143 and all3144 partially complements the capability of 
Escherichia coli ΔacrAB to transport ethidium. We postulate that the RND 
transporter All3143 and the predicted membrane fusion protein All3144, as 
homologs of E. coli AcrB and AcrA, respectively, are major players for 
antibiotic resistance in Anabaena sp. PCC 7120.