AlnA is a 35.77 kDa protein responsible for the hydrocarbon emulsifying and solubilizing activity of the Acinetobacter radioresistens KA53 bioemulsifier alasan. Deletion and substitution derivatives of AlnA were produced by site-directed polymerase chain reaction (PCR) mutagenesis and then used to study their ability to solubilize phenanthrene. AlnA contains four hydrophobic regions. Deletions of three or more amino acids from the hydrophobic N-terminus of AlnA caused a large decrease in solubilizing activity, whereas deletions from the C-terminus of 4, 7, 18 and 35 amino acids resulted in the loss of only 9, 22, 35 and 46% of the activity, respectively. Deletions of any of the three internal hydrophobic regions of AlnA caused a greater than 50% loss in solubilizing activity. The solubilizing activity of chimeric proteins, containing sequences from both AlnA and the homologous (but not surface active) Escherichia coli outer membrane protein A (OmpA), indicated that the most important sequences needed for solubilizing phenanthrene are on the N-terminal half of AlnA, especially the two hydrophobic loops (amino acids 37-45 and 164-171) on the β-barrel structure. Gel electrophoresis experiments demonstrated that AlnA and derivatives which retained high phenanthrene-solubilizing activity formed 210 kDa complexes in the presence of phenanthrene. The relationship between AlnA structure and its surface activity is discussed.