Contribution of gentamicin 2'-N-acetyltransferase to the O acetylation of peptidoglycan in Providencia stuartii

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Abstract
A collection of Providencia stuartii mutants which either underexpress or overexpress aac(2')-Ia, the chromosomal gene coding for gentamicin 2'-N-acetyltransferase (EC 2.3.1.59), have been characterized phenotypically as possessing either lower or higher levels of peptidoglycan O acetylation, respectively, than the wild type. These mutants were subjected to both negative-staining and thin-section electron microscopy. P. stuartii PR100, with 42% O acetylation of peptidoglycan compared with 52% O acetylation in the wild type, appeared as irregular rods. In direct contrast, P. stuartii strains PR50.LM3 and PR51, with increased levels of peptidoglycan O acetylation (65 and 63%, respectively), appeared as coccobacilli and chain formers, respectively. Membrane blebbing was also observed with the chain-forming strain PR51. Thin sectioning of this mutant indicated that it was capable of proper constriction and separation. P. stuartii PM1, when grown to mid-exponential phase, did not have altered peptidoglycan O-acetylation levels, and cellular morphology remained similar to that of wild-type strains. However, continued growth into stationary phase resulted in a 15% increase in peptidoglycan O acetylation concomitant with a change of some cells from a rod-shaped to a coccobacillus-shaped morphology. The fact that these apparent morphological changes were directly related to levels of O acetylation support the view that this modification plays a role in the maintenance of peptidoglycan structure, presumably through the control of autolytic activity.