TABLE 2.

Regulation of iron transport by signals in addition to iron availability^a

Environment	Most likely iron source(s)b	Environmental signalsc	Possible Vibrio regulator(s)	Effect(s) on Vibrio iron transporters
Marine surface waters	Fe3+ compounds	O2 present	Fnr,d ArcABd	↑ siderophore
		Neutral or alkaline pH	PepAe	↓ Feo
Sediments, solid surfaces, or biofilms	Fe2+ compounds	Low O2 concn, reduced diffusion, and crowding in biofilm	Fnr,d ArcABd	↑ Feo
			LitR,f quorum sensingf	↓ siderophore, ↓ heme transporters
Host	Heme and transferrin in blood and tissues, lactoferrin in secretions, and Fe2+ iron complexes in intestine	Temperature	H-NSd	↑ heme transporters
		Specific C sources	Crpf	↑ TonB
		Specific N sources in squid	GlnDf	↑ siderophore
		Low O2 concn in intestine	Fnr,d ArcABd	↓ siderophore, ↑ Feo
		Bile or antimicrobial peptides in intestine	Cpxf	↑ heme transporters, ↑ siderophore

^aShown is a summary of the effects of environmental conditions on the regulation of iron transport systems. Conditions that have been shown to affect specific iron transporters are discussed in the text. Arrows indicate a positive (up arrow) or negative (down arrow) effect on expression of the iron transport genes.

^bIron sources present in each environment are predicted based on the known distributions of iron complexes.

^cEnvironmental signals are predicted based on conditions known to exist in each environment.

^dRegulator that affects iron transport in other species and for which homologs are present in Vibrio spp.

^eRegulator that is present in Vibrio spp. and is known to regulate genes in response to the stated conditions but that has not been shown to regulate iron acquisition genes.

^fRegulator that is known to affect Vibrio iron transport.