Patterns of gene expression in Bacillus subtilis colonies

Bacillus subtilis 5:7, a derivative of macrofiber-producing strain FJ7, carries the lacZ reporter gene within Tn917 at an unknown location in the host genome. Expression of the host gene carrying lacZ within colonies of 5:7 was observed by examining growth under different conditions in the presence of 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal). At a high plating density small colonies arose that expressed the host gene early and throughout the colony, whereas at a low density large colonies were produced that expressed the host gene late in development and only in cells forming a ring pattern close to the colony periphery. A highly regulated spatial and temporal gene expression pattern was observed in growth from cross-streaks, suggesting that gene expression is responsive to concentration gradient fields established by neighboring growth. Colonies cultured on agar blocks revealed that expression was governed by depletion of a medium component and also by the geometry of the substrate upon which the colonies grew. At least three factors influenced the control of expression: (i) the concentration of a diffusible component of the medium exhausted by cell growth, (ii) a spatial-temporal factor related to growth within the colony, and (iii) the geometry of the growth substrate.