THE ATYPICAL CODON USAGE OF THE PLANT PSBA GENE MAY BE THE REMNANT OFAN ANCESTRAL BIAS

The psbA gene of the chloroplast genome has a codon usage that is unus ual for plant chloroplast genes. In the present study the evolutionary status of this codon usage is tested by reconstructing putative ances tral psbA sequences to determine the pattern of change in codon bias d uring angiosperm divergence. It is shown that the codon biases of the ancestral genes are much stronger than all extant flowering plant psbA genes. This is related to previous work that demonstrated a significa nt increase in synonymous substitution in psbA relative to other chlor oplast genes. It is suggested, based on the two lines of evidence, tha t the codon bias of this gene currently is not being maintained by sel ection. Rather, the atypical codon bias simply may be a remnant of an ancestral codon bias that now is being degraded by the mutation bias o f the chloroplast genome, in other words, that the psbA gene is not at equilibrium. A model for the evolution of selective pressure on the c odon usage of plant chloroplast genes is discussed.