The Frankia genome contains two truncated hemoglobin genes (hboN and hboO) whose functions remain to be determined. Nitric oxide (NO) generated by the addition of 400 microM SNAP (S-nitroso-N-acetylpenicillamine) caused a 10-fold increase in hboN gene expression but had no effect on hboO expression. The addition of the NO scavenger, carboxy-PT10, reduced the effect of SNAP. hboO gene expression increased under low-oxygen conditions, while hboN expression was unaffected. These results suggest that HboN may function in protection from nitrosative stress and that HboO may act as an oxygen transport molecule for increased respiration in hypoxic environments.