Investigation of the het genes that control heterokaryon incompatibility between members of heterokaryon-compatibility (h-c) groups A and G1 of Aspergillus nidulans

Abstract

Summary: The recombinant plasmid pTK6 is composed of a 13·6 kb fragment from pTR2030 encoding phage resistance determinants for restriction/modification (R⁺/M⁺) and abortive phage infection (Hsp⁺) cloned into shuttle vector pSA3 (erythromycin resistance, Em^r). Conjugal matings were performed to mobilize pTK6-encoded markers from Lactococcus lactis subsp. lactis MMS362 and MG1363. Em^r transconjugants were recovered at 10⁻⁶ per input donor and harboured pTK6 or recombinant plasmids not found in either parental strain. The recombinant plasmids (pTRK78 and pTRK79) encoded Em^r, Hsp⁺ and R⁺/M⁺, and transferred at high frequency in second-round matings. Mobilization of pTK6 from the otherwise plasmid-free donor, L. lactis MG1363, confirmed the presence of a conjugal element in this strain. Phage resistance in transconjugants containing pTRK78 and pTRK79 was markedly enhanced over pTK6-directed Hsp⁺ and R⁺/M⁺. In L. lactis LM2345 transconjugants, a reduction in plaque size was accompanied by a significant decrease in the efficiency of plaquing for phages c2 (10⁻² to 10⁻⁶) and p2 (< 10⁻⁹). L. lactis NCK203 transconjugants containing pTRK78 or pTRK79 exhibited an additional 100-1000-fold reduction in the plaquing efficiency of ø48 (10⁻⁴ to 10⁻⁵) over pTK6 imposed restriction (10⁻²). Increased resistance to phage was a consequence of the physical interaction of pTR2030-derived sequences on pTK6 with a conjugal element resident in the donor strains.