| contribution of phe-7 to tat-dependent export of β-lactamase in xanthomonas campestris. | strains of xanthomonas campestris pv. campestris isolated in taiwan are commonly resistant to ampicillin owing to the constitutive expression of a chromosomally encoded β-lactamase that is secreted into the periplasm. in this study, we found that levels of β-lactamase vary among x. campestris pv. campestris strains, a difference that can be attributed to amino acid substitutions at least at positions 7 and 206, with the former having the major impact. bioinformatic and pcr analyses indicated tha ... | 2012 | 22526303 |