Pii: S0378-1119(02)00503-6

Pseudomonas aeruginosa PA01 has a large (6.7 Mbp) genome with a high (67%) G 1 C content. Codon usage in this species is dominated by this compositional bias, with the average G 1 C content at synonymously variable third positions of codons being 83%. Nevertheless, there is some variation of synonymous codon usage among genes. The nature and causes of this variation were investigated using multivariate statistical analyses. Three trends were identified. The major source of variation was attributable to genes with unusually low G 1 C content that are probably due to horizontal transfer. A lesser trend among genes was associated with the preferential use of putatively translationally optimal codons in genes expressed at high levels. In addition, genes on the leading strand of replication were on average more G 1 T-rich. Our findings contradict the results of two previous analyses, and the reasons for the discrepancies are discussed. q 2002 Elsevier Science B.V. All rights reserved.